Most women are offered a detailed scan at about 20 weeks to see if the baby has developed normally; this is known as the anomaly scan. You will now be halfway through your pregnancy and your baby will be about 20cm long from head to toe, you will probably be feeling some of your baby’s movements.
Seeing your baby on a screen is a very exciting and emotional time; your partner and your other children are welcome to share the experience with you. Many couples want to know the sex of the baby and have some photos but before you have the scan, you should understand that its main purpose is to look for abnormalities.
What is examined at the 20-week scan?
Your baby’s internal organs are examined in cross-sectional views, or “slices” of your baby, which may be difficult — if not impossible — for you to make out! Bones will look white on the scan, fluid will be black and soft tissues will look grey.
The head is usually examined first. A normal head is rugby-ball shaped with a mid-line separating the two halves of the brain which are surrounded by fluid. There is a dumb-bell-shaped structure called the cerebellum (hindbrain) at the back of the head.
Spine and abdominal wall
The sonographer will check your baby’s spine in both the long view and in cross-section, moving through the neck and shoulders to the pelvis. He is making sure that all the vertebrae are in alignment, that the skin covers the spine at the back and the baby’s abdominal wall covers all the internal organs at the front.
Heart and stomach
We will look at the heart. The top two chambers, or “atria”, and the bottom two chambers, or “ventricles”, should be equal in size, and the valves should open and close with each heartbeat. She will then look beneath the diaphragm to see the stomach, under the heart on the left side. Your baby swallows some of the amniotic fluid that it lies in; this can be seen in the stomach as a black bubble.
Kidneys and bladder
The two kidneys are on either side of the spine, below the stomach, and are usually quite hard to see. If the baby’s bladder is full, it’s easy to see a black bubble in the pelvis.
Hands and feet
The long bones and position of hands and feet are examined; the fingers and toes are looked at but not counted.
Placenta, umbilical cord and amniotic fluid
The placenta may be on the front of the back wall of your womb, usually near the top (or fundus) so may be described as “fundal” on your scan report. Many are described as “low” because they reach down to or cover the neck of the womb (cervix). If your placenta is low, another scan will be arranged in the third trimester, by which time most placenta will have moved away from the cervix.
It is possible to count the three vessels in the umbilical cord but this may not be done routinely. There should be enough amniotic fluid surrounding the baby to allow it to move freely at this stage.
What is measured?
Measurements of your baby’s head circumference (HC) and diameter (biparietal diameter or (BPD) are taken, along with the abdominal circumference (AC) and femur or thigh bone (FL), to date the pregnancy and to make sure your baby is growing normally. If there is more than seven days’ difference between your scan dates and your dates according to your last monthly period (LMP) in the second trimester, the scan dates will be used from now on.
What abnormalities can be seen on scan at 20 weeks?
About half of all major abnormalities will be seen on a scan and half will not be seen. This means that even if your scan is normal, there is a small chance that your baby will still have a problem.
The following information is published by the Royal College of Obstetricians and Gynaecologists. It shows what chance there is of certain serious abnormalities (in the rare eventuality that your baby has one) being picked up at a 20-week scan:
• Anencephaly (absence of the top of the head) — 99 percent
• Exomphalos/gastroschisis (defects of the abdominal wall) — 90 percent
• Major limb abnormalities (missing or very short limbs) — 90 percent
• Spina bifida (open spinal cord) — 90 per cent
• Major kidney problems (missing or abnormal kidneys) — 85 percent
• Diaphragmatic hernia (hole in the muscle separating chest and abdomen) — 60 percent
• Hydrocephalus (excess fluid within the brain) — 60 percent
• Down’s syndrome (some babies with Down’s may have heart or bowel problems that may be seen on the scan) — 40 percent
• Major heart problems (defects of chambers, valves, or vessels) — 25 percent
• Autism — never seen
• Cerebral palsy — never seen.
Some problems may not be visible on a scan until later in your pregnancy. Heart defects may become more obvious as your baby gets bigger. Some bowel problems, such as obstructions, may not be visible until later.
At 20 weeks it is also the first time the baby hears her/his mother’s voice.
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