Obstetric Ultrasonography: An Overview

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21st Sep 2017 Health Reference this

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Obstetric ultrasonography is primarily a non-invasive prenatal diagnostic tool for visualizing the embryo or foetus in its mother’s womb. While performing an ultrasound scan, the obstetric sonographer assesses the location of the placenta, checks for multiple pregnancies, monitors foetal development, detects abnormalities and monitors the health of the mother. As a standard part of prenatal care, ultrasound scans can be done at any stage of pregnancy. However, the two main scan point used in obstetric ultrasound to monitor the development of the embryo / foetus, examine embryological features and identify pathologies are between 11 and 14 weeks and between 18 and 20 weeks. During obstetric ultrasound foetal morphology, skeletal features, amniotic volume, foetal heart function and vital organs are assessed for abnormalities. Detected pathologies and abnormalities can be treated or managed through medical interventions in utero, improved prenatal care and abortion in extreme cases.

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The nuchal translucency ultrasound is performed in the first trimester of pregnancy between 10 weeks 3 days and 13 weeks 6 days of pregnancy. This ultrasound scan is done to determine pregnancies that have higher risks of chromosomal abnormalities like Trisomy 21 (Down syndrome), 13 and 18. Nuchal translucency scan is done by measuring the thickness of a fluid layer deep to the skin at the back of the baby’s neck. Research has shown that increased thickness of the nuchal translucency is associated with an increased risk of Down syndrome and other chromosomal abnormalities. During the nuchal translucency scan, it is also possible to confirm pregnancy due date, diagnose multiple pregnancies, diagnose early pregnancy failure and check the normality of foetal anatomy.

The ultrasound scan at 18 to 20 weeks is also called morphology scan or foetal anomaly scan. It primarily checks for foetal structural development and growth abnormalities and position of the placenta among others. The areas examined during the scan are.

  1. Head, face and its internal structures: The head circumference and biparietal diameter are measured and the foetus’s face is examined to see if there is any cleft of the lip. The distance between the foetus’s eyes is measured (binocular distance).
  2. The spine: This is checked to ensure that it is fully covered by the skin and the vertebra are forming properly and in alignment
  3. The abdominal wall: This is checked to ensure that the internal organs are encased within the abdomen. The abdominal circumference is also measured to estimate the size of the foetus.
  4. The heart, chest and diaphragm: This is examined to see if the four chambers are of appropriate sizes and connected by valves which open and close with each heart beat. The main vessels connecting the heart are examined. Most foetuses have a heart rate between 120 -180 beats per minute.
  5. The stomach: This is examined to ensure that it is properly located below the heart and filled with some amniotic fluid.
  6. The kidneys and bladders: These are checked to ensure they are formed and functioning properly
  7. The arms, legs, hands and feet: These are checked to ensure they are properly formed. The femur and humerus are measured to check they are growing appropriately.
  8. The placenta’s: The position of the placenta in the uterus is recorded. If the lowest edge of the placenta is close to the cervix or lies over the cervix, it is termed low-lying or placenta praevia. The distance between the placenta and pelvis is measured and a repeat scan is recommended to ensure the placenta has moved away from the cervix.
  9. The cervix: The length of the cervix is important if there is a history of premature labour, vagina bleeding or pain.
  10. The uterus: This is checked for the presence of fibroids and its size and location.
  11. The umbilical cord: This is properly examined and the number of vessels within the cord is counted. Normally, there are only two arteries and one vein.
  12. The amniotic fluid: This is assessed to check that the amount is within normal range.

HIGHLIGHT AND DETAIL A FEW OF THE SIGNIFICNT EMBRYOLOGICAL DEVELOPMENTS THAT OCCUR PRIOR TO THE 11-14 WEEK SCAN AND BETWEEN THE TWO SCANS.

In week one the embryo floats freely in the uterus

In week two the embryo implants in the uterus

In week three the formation of 3 layer embryo commences. All tissues of the body are formed from these 3 embryonic tissue layers

  1. Ectoderm: this forms the central and peripheral nervous system and epithelium of the skin
  2. Mesoderm: this forms the body connective tissues; blood, bone, muscle, connective tissue, skin, gastrointestinal and respiratory tracts.
  3. Endoderm: This forms the gastrointestinal tract organs and the epithelium of the gastrointestinal and respiratory tracts.

In week four, the heart which is the first functioning organ is formed. Week 4 – 8 marks the early development of the other organs, tissues and limbs.

At the end of the first semester, all organs are formed,facial feature are rapidly forming, external sex organs are visible but positive sex identification is difficult, the neck is well-defines nail beds and tooth buds begin to form, the rudimentary kidneys excretr small amounts of irine into the amniotic sac. The fetus is 2.9 inches long and weighs about 14g

The first trimester covers the period from conception to the end of the 13th menstrual week. This is a time of dynamic growth and the differentiation and development of most organ systems. The embryo has the greatest risk of mal-development, injury, and death during this period because of external factors (infection, drugs, and radiation) or chromosome abnormalities.

WEEK 1

Azygoteis formed when a sperm fertilizes the ovum, usually in the ampullary portion of the fallopian tube. Cell division occurs as the zygote migrates to the endometrial cavity.

WEEK 2

Acystic structure called theblastocystis formed with two cell layers present. The outer cell layer is thetrophoblastthat forms the chorion and the fetal components of the placenta. The inner cell layer forms the embryo, umbilical cord, amnion and secondary yolk sac. The blastocyst implants 7-10 days after fertilization by burrowing into the endometrium. The blastocyst continues to develop forming a recognizable gestational sac completely covered by decidua.

WEEK 3

In week three the formation of 3 layer embryo commences. All tissues of the body are formed from these 3 embryonic tissue layers

  1. Ectoderm: this forms the central and peripheral nervous system and epithelium of the skin
  2. Mesoderm: this forms the body connective tissues; blood, bone, muscle, connective tissue, skin, gastrointestinal and respiratory tracts.
  3. Endoderm: This forms the gastrointestinal tract organs and the epithelium of the gastrointestinal and respiratory tracts.

WEEK 4

The early gestational sac is first visible by TV US at approximately 4.5 weeks menstrual age. A small cystic structure 2-3 mm in size is seen burrowed into and completely covered by echogenic decidua, giving it the appearance of a thick-walled cyst. The appearance of this very early gestational sac is called theintradecidual sign.

A 3-MM GESTATIONAL SIGN BURROWED WITHIN THE DECIDUA.

WEEK 5

The secondaryyolk sacis the first structure visible within the gestational sac and is a finding that unequivocally confirms identification of the gestational sac. The yolk sac is usually visible by TV US at the end of the fifth menstrual week when the GS measures 8-10 mm. It serves a primary source of nutrients for the embryo before placental function is established.

Theembryois first visualized as an echogenic, disk-like structure approximately 2 mm long within the amniotic cavity. The embryo is normally seen by the end of the fifth menstrual week. The normal embryo grows approximately 1 mm per day in length.

WEEK 6

Embryonic cardiac activitymay be seen with TV US when the embryo is as small as 1-2 mm. All normal embryos should have cardiac activity visible on TV US when the embryo measures 5 mm or more in length. Initial visible embryonic heart rate at 5-6 weeks GA is 100 beats/minute. The rate increases over the next 2-3 weeks to 140 beats/minute.

WEEK 7

The embryo’s hands and feet are shaped like paddles, but the fingers are beginning to take shape. The embryonic tail has now almost disappeared. The pituitary gland is also forming and the fetus is beginning to grow muscle fibers. The heart has divided into the right and left chambers and is beating about 150 beats a minute which is about twice the rate of an adult. The baby’s facial features are visible, including a mouth and tongue. The eyes have a retina and lens. The major muscle system is developed, and the baby starts to practice moving. The baby has its own blood type and the blood cells are produced by the liver.

WEEK 8

The unborn baby is now called a fetus. The fetus is protected by the amniotic sac and filled with fluid. Inside the child swims and moves gracefully. The fetus is now about 1/2 inch (12mm) long. The arms and legs have lengthened. During this time of development, the baby’s head appears much larger than the body because the brain is growing very rapidly. Brain waves can now be measured. The teeth and the palate are beginning to form and the larynx is developing. Through its parchment-thin skin, the baby’s veins are clearly visible. By the eighth week the ears begin to take shape.

WEEK 9

All of the organs, muscles, and nerves are in place and beginning to function. The eyelids have fused shut, As the hands and feet develop fingers andtoes, they lose their paddle-like look, and the touch pads on the fingers form.

WEEK 10

The fetus weighs about 1/3 of an ounce. The heart is almost completely developed and very much resembles that of a newborn baby. An opening the atrium of the heart and the presence of a bypass valve divert much of the blood away from the lungs, as the child’s blood is oxygenated through the placenta. The wrists and ankles have formed and the fingers and toes are clearly visible.Genitalshave begun to from, but it is too early to tell the sex of the fetus. By this week of the pregnancy the placenta has developed enough to support most of the critical job of producing hormones.

WEEK 11 -14

During this time, the vital organs which include the liver, kidneys, intestines, brain and lungs are almost fully formed and beginning to function. The vocal chords are complete and brain is fully formed. The eyes are slowly moving towards the centre of the face. The eyelids cover the eyes to protect the delicate optic nerve fibres. The head has hair and the fingers and toes develop soft nails, the baby’s unique fingerprints are in place. The ears are developing and the cheek bones are visible. The kidneys are developed and begin to secrete urine.

WEEK 15

The fetus has an adult’s taste buds and may be able to savor the mother’s meals. The baby’s thin skin is covered with ultra fine, downy hair (Lanugo) that usually disappears before birth

WEEK 16

The foetus weighs now three and half ounces and is 5 inches crown to rump. The baby can grasp with his hands, kick, or even somersault.

WEEK 17

The baby’s circulatory system and urinary tract are up and operating. The lungs are inhaling and exhaling amniotic fluid.

gestational

WEEK 4 SCAN: GESTATIONAL SAC 3-6MM

5weeks

WEEK 5 SCAN: GESTATIONAL SAC 6-12MM

week6

WEEK 6 SCAN: GESTATIONAL SAC 14mm to 25mm CRL (CROWN TO RUMP LENGTH) = 4mm to 7mm

 

7

WEEK 7 SCAN CRL (CROWN TO RUMP LENGTH) = 5mm to 12mm

eight

WEEK 8 SCAN CRL (CROWN TO RUMP LENGTH) = 16MM TO 20MM

Week

WEEK 9 SCAN CRL (CROWN TO RUMP LENGTH) = 20MM TO 30MM

Week

WEEK 10 SCAN CRL (CROWN TO RUMP LENGTH) = 31MM TO 41MM

Eleven

WEEK 11 SCAN CRL = 41MM TO 54MM

Week

WEEK 12 SCAN CRL= 54MM TO 71MM

13

WEEK 13 SCAN CRL= 74MM TO 87MM

OUTLINE AND DISCUSS THE RELEVANT ANATOMICAL KNOWLEDGE REQUIRED TO PERFORM SUCH ULTRASOUND ASSESSMENTS ACCURATELY ON YOUR PATIENTS. THIS SHOULD INCLUDE DETAILS OF SURFACE LANDMARKS AND THEIR RELATIONSHIP TO KEY INTERNAL ANATOMICAL STRUCTURES/FEATURES. INCLUDE EXAMPLES OF PATHOLOGIES TO ILLUSTRATE THE IMPORTANCE OF THE DEVELOPMENTAL PROCESS YOU HAVE CHOSEN

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As a Sonographer, it is important to have a sound knowledge and understanding of anatomy of the female pelvis to competently assess the relationship between the position of foetus and relevant landmarks of the pelvis and recognize any deviations from normal. During pregnancy, many morphological changes occur in the female reproductive system and associated abdominal structures. The physiological changes that take place during the course of pregnancy cause alterations in the composition of the pelvis, its shape, and the plane of inclination and internal dimensions of the true pelvis. All of these changes serve to support the pregnant uterus throughout the term of pregnancy and assist with the normal mechanisms of childbirth. The uterus enlarges to accommodate the developing fetus and placenta, and various alterations take place in the pelvic walls floor and contents which allow for expansion and which anticipates parturition. The uterus grows dramatically during pregnancy, increasing in weight from about 50g at the beginning of pregnancy to up to 1kg at term. Most of the weight gain is the result of increased vascularity and fluid retention in the myometrium. The myometrium thins with asvancing gestation from 2-3cm thick in early pregnancy to 1-2cm at term. The upper third of the cervix (isthmus) is gradually taken up into the uterine body during the second month to form the lower segment. The isthmus hypertrophies like the uterine body during the first trimester and triples in length to about 3cm. From the second trimester, the wall of the isthmus and that of the body are the same thickness and their junction is no longer visible externally.

Obstetric ultrasonography is primarily a non-invasive prenatal diagnostic tool for visualizing the embryo or foetus in its mother’s womb. While performing an ultrasound scan, the obstetric sonographer assesses the location of the placenta, checks for multiple pregnancies, monitors foetal development, detects abnormalities and monitors the health of the mother. As a standard part of prenatal care, ultrasound scans can be done at any stage of pregnancy. However, the two main scan point used in obstetric ultrasound to monitor the development of the embryo / foetus, examine embryological features and identify pathologies are between 11 and 14 weeks and between 18 and 20 weeks. During obstetric ultrasound foetal morphology, skeletal features, amniotic volume, foetal heart function and vital organs are assessed for abnormalities. Detected pathologies and abnormalities can be treated or managed through medical interventions in utero, improved prenatal care and abortion in extreme cases.

The nuchal translucency ultrasound is performed in the first trimester of pregnancy between 10 weeks 3 days and 13 weeks 6 days of pregnancy. This ultrasound scan is done to determine pregnancies that have higher risks of chromosomal abnormalities like Trisomy 21 (Down syndrome), 13 and 18. Nuchal translucency scan is done by measuring the thickness of a fluid layer deep to the skin at the back of the baby’s neck. Research has shown that increased thickness of the nuchal translucency is associated with an increased risk of Down syndrome and other chromosomal abnormalities. During the nuchal translucency scan, it is also possible to confirm pregnancy due date, diagnose multiple pregnancies, diagnose early pregnancy failure and check the normality of foetal anatomy.

The ultrasound scan at 18 to 20 weeks is also called morphology scan or foetal anomaly scan. It primarily checks for foetal structural development and growth abnormalities and position of the placenta among others. The areas examined during the scan are.

  1. Head, face and its internal structures: The head circumference and biparietal diameter are measured and the foetus’s face is examined to see if there is any cleft of the lip. The distance between the foetus’s eyes is measured (binocular distance).
  2. The spine: This is checked to ensure that it is fully covered by the skin and the vertebra are forming properly and in alignment
  3. The abdominal wall: This is checked to ensure that the internal organs are encased within the abdomen. The abdominal circumference is also measured to estimate the size of the foetus.
  4. The heart, chest and diaphragm: This is examined to see if the four chambers are of appropriate sizes and connected by valves which open and close with each heart beat. The main vessels connecting the heart are examined. Most foetuses have a heart rate between 120 -180 beats per minute.
  5. The stomach: This is examined to ensure that it is properly located below the heart and filled with some amniotic fluid.
  6. The kidneys and bladders: These are checked to ensure they are formed and functioning properly
  7. The arms, legs, hands and feet: These are checked to ensure they are properly formed. The femur and humerus are measured to check they are growing appropriately.
  8. The placenta’s: The position of the placenta in the uterus is recorded. If the lowest edge of the placenta is close to the cervix or lies over the cervix, it is termed low-lying or placenta praevia. The distance between the placenta and pelvis is measured and a repeat scan is recommended to ensure the placenta has moved away from the cervix.
  9. The cervix: The length of the cervix is important if there is a history of premature labour, vagina bleeding or pain.
  10. The uterus: This is checked for the presence of fibroids and its size and location.
  11. The umbilical cord: This is properly examined and the number of vessels within the cord is counted. Normally, there are only two arteries and one vein.
  12. The amniotic fluid: This is assessed to check that the amount is within normal range.

HIGHLIGHT AND DETAIL A FEW OF THE SIGNIFICNT EMBRYOLOGICAL DEVELOPMENTS THAT OCCUR PRIOR TO THE 11-14 WEEK SCAN AND BETWEEN THE TWO SCANS.

In week one the embryo floats freely in the uterus

In week two the embryo implants in the uterus

In week three the formation of 3 layer embryo commences. All tissues of the body are formed from these 3 embryonic tissue layers

  1. Ectoderm: this forms the central and peripheral nervous system and epithelium of the skin
  2. Mesoderm: this forms the body connective tissues; blood, bone, muscle, connective tissue, skin, gastrointestinal and respiratory tracts.
  3. Endoderm: This forms the gastrointestinal tract organs and the epithelium of the gastrointestinal and respiratory tracts.

In week four, the heart which is the first functioning organ is formed. Week 4 – 8 marks the early development of the other organs, tissues and limbs.

At the end of the first semester, all organs are formed,facial feature are rapidly forming, external sex organs are visible but positive sex identification is difficult, the neck is well-defines nail beds and tooth buds begin to form, the rudimentary kidneys excretr small amounts of irine into the amniotic sac. The fetus is 2.9 inches long and weighs about 14g

The first trimester covers the period from conception to the end of the 13th menstrual week. This is a time of dynamic growth and the differentiation and development of most organ systems. The embryo has the greatest risk of mal-development, injury, and death during this period because of external factors (infection, drugs, and radiation) or chromosome abnormalities.

WEEK 1

Azygoteis formed when a sperm fertilizes the ovum, usually in the ampullary portion of the fallopian tube. Cell division occurs as the zygote migrates to the endometrial cavity.

WEEK 2

Acystic structure called theblastocystis formed with two cell layers present. The outer cell layer is thetrophoblastthat forms the chorion and the fetal components of the placenta. The inner cell layer forms the embryo, umbilical cord, amnion and secondary yolk sac. The blastocyst implants 7-10 days after fertilization by burrowing into the endometrium. The blastocyst continues to develop forming a recognizable gestational sac completely covered by decidua.

WEEK 3

In week three the formation of 3 layer embryo commences. All tissues of the body are formed from these 3 embryonic tissue layers

  1. Ectoderm: this forms the central and peripheral nervous system and epithelium of the skin
  2. Mesoderm: this forms the body connective tissues; blood, bone, muscle, connective tissue, skin, gastrointestinal and respiratory tracts.
  3. Endoderm: This forms the gastrointestinal tract organs and the epithelium of the gastrointestinal and respiratory tracts.

WEEK 4

The early gestational sac is first visible by TV US at approximately 4.5 weeks menstrual age. A small cystic structure 2-3 mm in size is seen burrowed into and completely covered by echogenic decidua, giving it the appearance of a thick-walled cyst. The appearance of this very early gestational sac is called theintradecidual sign.

A 3-MM GESTATIONAL SIGN BURROWED WITHIN THE DECIDUA.

WEEK 5

The secondaryyolk sacis the first structure visible within the gestational sac and is a finding that unequivocally confirms identification of the gestational sac. The yolk sac is usually visible by TV US at the end of the fifth menstrual week when the GS measures 8-10 mm. It serves a primary source of nutrients for the embryo before placental function is established.

Theembryois first visualized as an echogenic, disk-like structure approximately 2 mm long within the amniotic cavity. The embryo is normally seen by the end of the fifth menstrual week. The normal embryo grows approximately 1 mm per day in length.

WEEK 6

Embryonic cardiac activitymay be seen with TV US when the embryo is as small as 1-2 mm. All normal embryos should have cardiac activity visible on TV US when the embryo measures 5 mm or more in length. Initial visible embryonic heart rate at 5-6 weeks GA is 100 beats/minute. The rate increases over the next 2-3 weeks to 140 beats/minute.

WEEK 7

The embryo’s hands and feet are shaped like paddles, but the fingers are beginning to take shape. The embryonic tail has now almost disappeared. The pituitary gland is also forming and the fetus is beginning to grow muscle fibers. The heart has divided into the right and left chambers and is beating about 150 beats a minute which is about twice the rate of an adult. The baby’s facial features are visible, including a mouth and tongue. The eyes have a retina and lens. The major muscle system is developed, and the baby starts to practice moving. The baby has its own blood type and the blood cells are produced by the liver.

WEEK 8

The unborn baby is now called a fetus. The fetus is protected by the amniotic sac and filled with fluid. Inside the child swims and moves gracefully. The fetus is now about 1/2 inch (12mm) long. The arms and legs have lengthened. During this time of development, the baby’s head appears much larger than the body because the brain is growing very rapidly. Brain waves can now be measured. The teeth and the palate are beginning to form and the larynx is developing. Through its parchment-thin skin, the baby’s veins are clearly visible. By the eighth week the ears begin to take shape.

WEEK 9

All of the organs, muscles, and nerves are in place and beginning to function. The eyelids have fused shut, As the hands and feet develop fingers andtoes, they lose their paddle-like look, and the touch pads on the fingers form.

WEEK 10

The fetus weighs about 1/3 of an ounce. The heart is almost completely developed and very much resembles that of a newborn baby. An opening the atrium of the heart and the presence of a bypass valve divert much of the blood away from the lungs, as the child’s blood is oxygenated through the placenta. The wrists and ankles have formed and the fingers and toes are clearly visible.Genitalshave begun to from, but it is too early to tell the sex of the fetus. By this week of the pregnancy the placenta has developed enough to support most of the critical job of producing hormones.

WEEK 11 -14

During this time, the vital organs which include the liver, kidneys, intestines, brain and lungs are almost fully formed and beginning to function. The vocal chords are complete and brain is fully formed. The eyes are slowly moving towards the centre of the face. The eyelids cover the eyes to protect the delicate optic nerve fibres. The head has hair and the fingers and toes develop soft nails, the baby’s unique fingerprints are in place. The ears are developing and the cheek bones are visible. The kidneys are developed and begin to secrete urine.

WEEK 15

The fetus has an adult’s taste buds and may be able to savor the mother’s meals. The baby’s thin skin is covered with ultra fine, downy hair (Lanugo) that usually disappears before birth

WEEK 16

The foetus weighs now three and half ounces and is 5 inches crown to rump. The baby can grasp with his hands, kick, or even somersault.

WEEK 17

The baby’s circulatory system and urinary tract are up and operating. The lungs are inhaling and exhaling amniotic fluid.

gestational

WEEK 4 SCAN: GESTATIONAL SAC 3-6MM

5weeks

WEEK 5 SCAN: GESTATIONAL SAC 6-12MM

week6

WEEK 6 SCAN: GESTATIONAL SAC 14mm to 25mm CRL (CROWN TO RUMP LENGTH) = 4mm to 7mm

 

7

WEEK 7 SCAN CRL (CROWN TO RUMP LENGTH) = 5mm to 12mm

eight

WEEK 8 SCAN CRL (CROWN TO RUMP LENGTH) = 16MM TO 20MM

Week

WEEK 9 SCAN CRL (CROWN TO RUMP LENGTH) = 20MM TO 30MM

Week

WEEK 10 SCAN CRL (CROWN TO RUMP LENGTH) = 31MM TO 41MM

Eleven

WEEK 11 SCAN CRL = 41MM TO 54MM

Week

WEEK 12 SCAN CRL= 54MM TO 71MM

13

WEEK 13 SCAN CRL= 74MM TO 87MM

OUTLINE AND DISCUSS THE RELEVANT ANATOMICAL KNOWLEDGE REQUIRED TO PERFORM SUCH ULTRASOUND ASSESSMENTS ACCURATELY ON YOUR PATIENTS. THIS SHOULD INCLUDE DETAILS OF SURFACE LANDMARKS AND THEIR RELATIONSHIP TO KEY INTERNAL ANATOMICAL STRUCTURES/FEATURES. INCLUDE EXAMPLES OF PATHOLOGIES TO ILLUSTRATE THE IMPORTANCE OF THE DEVELOPMENTAL PROCESS YOU HAVE CHOSEN

As a Sonographer, it is important to have a sound knowledge and understanding of anatomy of the female pelvis to competently assess the relationship between the position of foetus and relevant landmarks of the pelvis and recognize any deviations from normal. During pregnancy, many morphological changes occur in the female reproductive system and associated abdominal structures. The physiological changes that take place during the course of pregnancy cause alterations in the composition of the pelvis, its shape, and the plane of inclination and internal dimensions of the true pelvis. All of these changes serve to support the pregnant uterus throughout the term of pregnancy and assist with the normal mechanisms of childbirth. The uterus enlarges to accommodate the developing fetus and placenta, and various alterations take place in the pelvic walls floor and contents which allow for expansion and which anticipates parturition. The uterus grows dramatically during pregnancy, increasing in weight from about 50g at the beginning of pregnancy to up to 1kg at term. Most of the weight gain is the result of increased vascularity and fluid retention in the myometrium. The myometrium thins with asvancing gestation from 2-3cm thick in early pregnancy to 1-2cm at term. The upper third of the cervix (isthmus) is gradually taken up into the uterine body during the second month to form the lower segment. The isthmus hypertrophies like the uterine body during the first trimester and triples in length to about 3cm. From the second trimester, the wall of the isthmus and that of the body are the same thickness and their junction is no longer visible externally.

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