20 weeks

Most women will be offered an obstetric ultrasound scan at around 20 weeks gestation. The purpose of the 18 to 20 week scan is to check that the baby is developing normally. This is usually called the fetal morphology ultrasound scan, or the fetal anomaly scan.

Seeing your baby on a screen is a very exciting and emotional time. Your partner and your other children will be welcomed by most hospitals and clinics to share the experience with you. Many couples want to know the sex of their baby and have some photos. It's important to remember, though, that this scan's main purpose is to look for abnormalities.

What will I be able to see?

If you haven't already had a scan in this pregnancy, the sonographer will check that there is only one baby, and confirm your due date. She will point out the heartbeat and parts of the baby like the face and hands, before looking in detail at your baby. At this point, some sonographers will turn the screen away for the rest of the scan and show you views again at the end. The scan takes about 15-20 minutes.

What is examined at the 20 week scan?

Your baby's internal organs are examined in cross-sectional views which may be difficult for you to make out. Bones appear white on the scan, fluid is black, and soft tissues look grey and speckled.

• The head is usually examined first, to check its shape and internal structures. Some severe brain problems are visible at this stage. The sonographer will look at the baby's face to see if she can see any cleft of the lip, but clefts of the palate inside of the baby's mouth are difficult to see and are not often picked up on scans.
• The spine is checked in both the long view and in cross section, to make sure that all the vertebrae are in alignment and that the skin covers the spine at the back. The baby's abdominal wall is also checked to make sure it covers all the internal organs at the front.
• The heart is looked at for its size and shape. 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. Some hospitals offer a more detailed scan of the heart to look at the major blood vessels. Whether your hospital offers this level of scan depends on the quality of the ultrasound machines, the time available for each scan and on whether the sonographers have had special training.
• The stomach should be visible below the heart. Your baby swallows some of the amniotic fluid that it lies in, which is seen in the stomach as a black bubble.
• The sonographer will check that your baby has two kidneys, and that urine flows freely into the bladder. If the bladder is empty it should fill up during the scan and be easy to see - your baby has been weeing every half an hour or so for some months now!
• Hands and feet are examined and the fingers and toes are looked at.
• The placenta, umbilical cord and amniotic fluid are all checked. The placenta may be on the front or 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. (Read more about having a low-lying placenta.)

It is possible to count the three blood vessels in the umbilical cord. There should be enough amniotic fluid surrounding the baby to allow it to move freely at this stage.

What other details might be checked?

If you have a history of premature labour or late miscarriage, a scan to measure the length of your cervix may be performed, though this is more accurately done via a vaginal ultrasound.

What abnormalities can be seen on a scan at 20 weeks?

This scan is carried out in order to look for any abnormalities in the baby's structural development and growth, such as spina bifida or heart anomalies, and to check the position of the placenta.

About half of all major abnormalities will be seen on a scan and half will not be seen. So even if your scan is normal, there is a small chance that your baby will still have a problem.

Some problems, such as heart defects and bowel obstructions, may not be visible until later in your pregnancy.