The Anatomy Scan: 18-20 week ultrasound

At around 5 months from your LMP (last menstrual period),
your doctor or midwife will send you for an ultrasound. At this scan, you can
find out the sex IF you choose. Keep in mind that there are some areas and some
countries where it is policy not to reveal gender, but generally, this is the
scan where the sex can be reliably determined. Click for more about how we see boy versus girl Many people look most forward to this scan because of the possibility of finding out the gender, but there are so many more important reasons to do this scan. All of the baby’s anatomy from head to toe will be carefully examined, and lots of measurements are taken to ensure normal growth. In order to see the anatomy that lies beneath the surface of the skin inside of your baby’s body, the images will be taken in black and white 2D (rather than those fancy 3D images).

Click for tips on how to read and understand your 20 week ultrasound.


This scan is very important since it has been repeatedly demonstrated in multiple studies across different countries that performing an anatomy scan makes a difference in overall rates of infant mortality. Identifying fetuses with an anatomical abnormality before birth allows for delivery of the infant in the proper hospital environment with a prepared team of doctors and professionals who can give your baby the best possible care. For example, if you live in a remote area, knowing that your baby has a serious physical abnormality before it is born means you can plan to give birth in a hospital that has specialized neonatal care unit with experts who are equipped and prepared to handle your baby’s special needs.

20 week 3D image

20 week 3D image

Because there is so much to examine, this scan can take quite a while. In addition, sometimes the  baby simple doesn’t cooperate and insists on lying in a position that makes visualization of some structures impossible. There is no bad position for the baby at this point, but there are bad positions for getting good photos!! Occasionally, you may need to come back later in the day or at another appointment to get a complete exam.

What things do the doctors check on this ultrasound?

  • The fetal brain structures are thoroughly examined to ensure that they appear normal and there are no blockages or
  • The fetal face, eyes,  lips, skull, and jaws are imaged, ensuring that everything appears normal and there is no cleft lip.
  • Internal organs such as the kidneys, diaphragm, stomach, bladder, thorax, and intestines are examined. Blockages or defects in
    these areas can be identified and, in some cases, some interventions can be performed if blockages are seen. For example, if kidney or bladder blockages are seen, doctors can place a thin drainage tube which can prevent damage to the organs
    until proper surgical repair can be done after birth.
  • The spine is examined to make sure the skin is closed and there is no evidence of spina bifida (an opening in the spine) or
    other problems.
  • The arms, legs, hands, and feet will be examined. It is hard to account for each individual finger and toe, but the
    general structure of the hands and feet is checked.
  • The umbilical cord will be checked to ensure that it enters the abdomen normally and that it has 3 vessels. The umbilical cord
    should have 2 arteries and 1 vein. Sometimes only 1 artery and 1 vein is seen (known as a 2 vessel cord). This is not a terrible thing but follow up ultrasounds will be needed to ensure that the baby is growing well.
  • The baby’s arm and leg bones, head, abdomen, and bones are measured which allows an assessment of the overall growth of the baby.
  • Fetal heart: many images of the heart and the main vessels are examined. These images can sometimes take a while since the
    heart is a moving structure and getting a thorough exam can often be difficult. If the baby’s position prevents a good evaluation or a possible abnormality is suspected, you may be referred for a fetal echocardiogram. Fetal echocardiography is an exam performed  by a doctor who specializes in prenatal and infant hearts. He or she can best evaluate and address any potential problems that might be found. Rest assured, most of the time, the exam turns out to be normal.

In addition to looking carefully at the baby, the uterus, amniotic fluid, and cervix will be checked. The cervix should appear long and
closed. If it is doesn’t appear as expected, this can alert your doctor to the possibility that you are at higher risk for pre-term labor. Follow up examinations may be required and, if it is decided that the cervix is indeed  beginning to dilate and open, a procedure known as a cerclage (click for more info) can be performed where the doctor inserts a stitch to hold the cervix closed.

The placenta will also be examined and the location documented. The only time the placental location is a concern is if it lies at
the cervix. If it is completely covering the cervix, it is known as a placenta previa. If the edge of the placenta is at or near the cervix, this is known as a low-lying placenta. For more information, click here.

Because the placenta is a vascular structure that is embedded in the uterine wall by a multitude of blood vessels, if it lies at the cervix and the cervix begins to dilate, it can cause a severe life-threatening emergency for mother and baby. Fortunately, true previas are uncommon and occur in only about 5% of pregnancies. However, it is not uncommon at this 18-20 week scan to be told
that the placenta is low-lying and needs to be followed up with another ultrasound at a later date. Try not to worry since most of the time, a low-lying placenta will migrate away from the cervix as the uterus expands and will not be a concern by the third trimester.

Hopefully, the baby will be in a good position and you will get some good photos for you to take home with you. If the baby is cooperating and your ultrasound center has the capabilities, you may get some 3D/4D pictures as well. However, babies are skinny at this time in development and often, the 2D profiles are the cutest. Click for more on the limitations of 3D/4D and to view a 20week 4D video.

Below is a video of a 2D 20 week fetus in a beautiful, face-up position. Here you can see it flexing and moving, and the heart can be seen as the area of fluttering motion in the chest.

Click for more tips on understanding your ultrasound.


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