Your 20-week Scan

This second-trimester ultrasound, typically done between 18–22 weeks, looks in detail at how your baby’s major organs and body systems have developed, as well as checking the placenta and the volume of the amniotic fluid.

What the scan reveals

By 20 weeks, your baby’s organs and body systems are well developed and can be seen clearly on an ultrasound scan. The sonographer performing the scan will look closely at how your baby’s major organs and body systems have formed and whether there are any indications of a problem (see Your baby’s checkups). In the majority of cases, the scan will reassure women that their baby is developing normally.

If your baby is found to have a problem, the sonographer will refer you to a maternal medicine expert who will confirm the findings and offer follow-up scans throughout the rest of your pregnancy. He or she will also talk to a pediatrician to ensure that they have enough information to take care of your baby at birth.

A picture of your 20-week scan shows the incredible detail visible on an ultrasound at this stage of your pregnancy.

Your due date

If you had a scan in the first trimester, your dates are unlikely to be changed after a second trimester scan. This is because dating is most accurate in early pregnancy when all babies essentially grow at the same rate. Later on, individual differences in growth start to appear, making it harder to tell if your baby is just big for dates or if you are further along than thought.

However, if you didn’t have a first trimester scan, the sonographer may change your dates if your baby is 10–14 days smaller or larger than expected. If your dates have been confirmed by a first trimester scan, a substantial lag may be a sign of a growth problem in your baby , although this is rare.

Measuring your baby

Because your whole baby no longer fits on the screen, the crown–rump length won’t be measured. Instead, your baby’s size will be calculated by combining a series of measurements in a mathematical formula. The sonographer will measure the width (biparietal diameter) and circumference of your baby’s head, the circumference of your baby’s tummy (abdominal circumference), and the length of your baby’s upper leg bone (femur length). These measurements help estimate the size of your baby and check that this is within the normal range for this stage of pregnancy.

Placenta and amniotic fluid

The placenta will be examined to ensure that it appears normal and isn’t blocking the baby’s exit route (the cervix). It’s quite common in early pregnancy for the placenta to be low-lying and to cover the cervix, but in 95 percent of cases, the placenta moves up and out of the way by the third trimester as the uterus grows.

If your placenta is low lying, your doctor will arrange a follow-up scan later in pregnancy to check that the placenta has moved up out of the way. If a later scan shows that it has failed to move up, a condition known as placenta previa , you will be monitored until the birth.

The amniotic fluid is assessed to check that there isn’t too little or too much. If there is too much fluid, it may be possible to drain some using amniocentesis to decrease the risk of later complications such as premature labor. Too little fluid can indicate a problem with fetal growth or your baby’s renal tract and your baby may need to be monitored.

Markers for Down syndrome

Although ultrasound at this stage is not a reliable way to detect Down syndrome, it can pick up certain signs known as “soft markers” that may suggest that your baby has an increased risk of Down syndrome. However, many markers are very common and are not usually a cause for alarm unless you already have an increased risk for Down syndrome, for example, because you are over 35 years of age, or were given a high risk from a previous screening test. Common markers include a bright spot in your baby’s heart, seen in about 1–2 percent of normal babies; extra fluid in your baby’s kidneys; short leg or arm bones; thickened neck skin (nuchal fold); and bright (echogenic) fetal intestines.

Some specific abnormalities are linked with a far higher risk of Down syndrome or other chromosomal abnormalities. These include certain kinds of heart defects and other major malformations, such as abnormalities in the spleen.

What happens next

If the sonographer detects soft markers, he or she will discuss the findings and may suggest a diagnostic test, especially if you already have an increased risk of Down syndrome. However, an earlier all-clear from a diagnostic test means that you can be confident your baby’s chromosomes are normal. If an obvious abnormality is seen, and you haven’t had a diagnostic test, this will probably be strongly advised. Having information from the scan will help you decide whether you want to proceed with a test. Whatever your decision, any associated abnormalities will be monitored throughout your pregnancy.

Your baby’s checkups
What the ultrasound looks at

During this scan, your baby’s organs are examined in detail and therefore it can take a little longer to perform than previous scans. For most, the scan provides reassurance that their baby is developing normally. The following areas will be checked.

  • The brain, including the fluid-filled spaces inside the brain and the shape of the back of your baby’s brain (cerebellum).

  • The spine, to check for spina bifida or other problems.

  • The upper lip, to check for cleft lip.

  • The heart, to rule out any major heart malformations. The heartbeat will be checked too.

  • The stomach and diaphragm.

  • The kidneys and bladder, making sure that both kidneys are present and there are no blockages or malformations.

  • The abdominal wall, to look for a defect, known as gastroschisis.

  • Your baby’s limbs, to make sure there are no hand or feet malformations, such as club foot.

  • The umbilical cord, to check that this has a normal number of blood vessels.

How the scan is interpreted

During your ultrasound, a picture of your baby is produced when high-frequency sound waves bounce off your baby and translate into an image on screen. In this image, solid matter, such as bones, are white, while softer tissue appears gray. Areas that contain fluid, such as blood vessels or the stomach, as well as the amniotic fluid, do not respond to the sound waves and therefore appear as black areas on the scan. The sonographer will study these details to assess how your baby is developing in the uterus.

The skull is well developed by now, and features such as the ears are clearly seen.

The four chambers of the heart can be identified and certain defects spotted.

The leg bones are measured to assess how your baby is growing.

Each vertebra in your baby’s spine will be counted to check for spina bifida.

Boy or girl?

Unless your baby is curled up or the legs are closed tight, the sonographer will have a pretty good idea of whether you’re going to have a boy or a girl. If you don’t want to know the sex, tell the sonographer at the start of the scan. If you do, consider asking the sonographer to point out the baby’s genitalia onscreen. If you are told which sex your baby is, resist the urge to go out and spend money on all-blue or all-pink decor since sonographers have been known to get it wrong!

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