Second Trimester Morphology


A second trimester morphology ultrasound is performed in the middle part of the pregnancy, usually between 18 and 20 weeks gestation.

All pregnant women have this ultrasound as a routine part of their antenatal (pregnancy) care. This ultrasound will check that your baby is growing and developing normally.

A second trimester ultrasound is performed with a transabdominal ultrasound.

Transabdominal ultrasound involves scanning through your lower abdomen. A small amount of ultrasound gel is put on the skin of the lower abdomen, with the ultrasound probe then scanning through this gel. The gel helps improve contact between the probe and your skin.

Sometimes a transvaginal ultrasound is also needed during a second trimester ultrasound. This may be to check for a low lying placenta, to look at the length of the cervix or there may be another indication to have this type of scan.

Transvaginal ultrasound during pregnancy, including the second trimester, is safe and will not harm either you or your baby. Your sonographer will be experienced at performing these ultrasounds during pregnancy.

Transvaginal ultrasound is an internal ultrasound. It involves scanning with the ultrasound probe lying in the vagina.

The transvaginal ultrasound probe is thin, about 2cm diameter. The probe is covered with a disposable protective sheath. A small amount of ultrasound gel is placed on the end of this probe. The probe is then gently inserted a short distance into the vagina by the sonographer. All transvaginal probes have been cleaned and sterilised according to recommended protocols.

Performing the transvaginal ultrasound usually causes less discomfort than a pap smear. No analgesia is required for this ultrasound.

Your privacy will always be respected during your ultrasound, especially the transvaginal examination. You will have a large towel covering your lower body, in addition to wearing a gown during the transvaginal ultrasound.

You will always have a choice about whether transvaginal ultrasound is performed. If you have concerns about transvaginal ultrasound, please discuss this with your sonographer.

We usually get better images during transabdominal ultrasound if the bladder is partially filled, so to help your examination we ask you to drink water prior to the assessment. Please empty your bladder 1 hour before your appointment, drink 2 glasses of water and try not to empty your bladder again until after your appointment.

A full bladder moves bowel out from the pelvis into the abdomen, helping visualisation of the pregnancy, uterus and cervix.

Your bladder should not be so full that it causes pain. If your bladder is very full and painful, you should empty a small amount so you are more comfortable.

You will be able to empty your bladder after the transabdominal ultrasound is completed and before the transvaginal ultrasound begins (if transvaginal ultrasound is required).

As the baby grows during the pregnancy, the uterus becomes larger and heavier. Lying down for this ultrasound can sometimes cause expectant mothers to become faint and nauseous, although this tends to be more of a problem in the third trimester or with a multiple pregnancy. This faint feeling is due to a fall in blood pressure, caused by compression of the veins that run at the back of the mother’s abdomen (the inferior vena cava) by the uterus and baby.

Please tell the sonographer if you begin to feel faint or nauseous at all during your scan. It is no trouble to change your position. Your position can be changed before you feel worse – lying more on your side or more upright will usually help resolve your symptoms.

It is routine practice for all pregnant women in Australia to have a second trimester morphology ultrasound at 18-20 weeks gestation.

For some women, this is the only ultrasound they have during their pregnancy, although most women also have scans earlier (nuchal translucency ultrasound) and later in the pregnancy (third trimester ultrasound).

The second trimester morphology ultrasound is a detailed scan of your developing baby. There are many aspects of the pregnancy that the sonographer will assess during this ultrasound to ensure that your baby is developing normally.

  • The baby’s anatomy or structure. We routinely look at the baby’s head and brain; face including lips and profile; heart; chest including diaphragm; abdomen including kidneys, bladder and stomach; spine; and limbs including hands and feet. Assessing all these parts of the baby requires a certain level of “cooperation” from the baby, as well as good scanning conditions. If your baby is not cooperative (mostly related to its position) or other factors make your ultrasound technically difficult (such as the size of the mother’s abdomen), we may require more images to be confident that all is well with baby. These images may be obtained later on the same day, or sometimes you may be asked to come back in a few weeks for another review.
  • Measurements of the baby. The second trimester ultrasound will commonly measure
    • baby’s head – biparietal diameter (BPD) and head circumference (HC).
    • baby’s  abdomen – abdominal circumference (AC).
    • baby’s leg – femur length ( FL).
    • baby’s arm – humerus length (HL)

Each measurement is compared to a normal reference range, which varies with gestation. Every individual baby has its own characteristics (for example, some babies have bigger head measurements or shorter femur measurements). These characteristics are often similar to the baby’s parents (for example, one parent has a larger than average head size). Measurements outside the normal range are not always significant, especially if the difference is minimal. Your baby will be carefully assessed if there are concerns about significant deviations from normal.

These measurements of baby are combined to give an average gestational age of your baby. By the time of this ultrasound, there can be up to 10 days difference between the expected gestational age of your baby (the number of weeks calculated using your last menstrual period or due date from an earlier ultrasound) and the calculated average ultrasound age. For example, if your baby should be 19 weeks gestation by your dates, we are usually not concerned if the baby measures either 10 days smaller or 10 days bigger on your ultrasound.

We tend not to focus on the estimated weight of your baby at this early stage of the pregnancy. The estimated weight of the baby is more important later in the pregnancy, or if premature delivery is imminent.

  • The baby’s heart rate and rhythm. Your baby’s heart rate will vary, just as it does in adults. Most babies have a heart rate between 120-180 beats per minute.
  • The number of babies. For women who have not had an early ultrasound, this may be the first opportunity to diagnose multiple pregnancy.
  • The position of the placenta. The sonographer will look at the position of the placenta in relation to the cervix. If the lowest edge (inferior margin) of the placenta is very close to the cervix, or lies over the cervix, it is termed “low-lying” or “placenta praevia”. About 2-3% of pregnancies have a low lying placenta on the second trimester morphology ultrasound. As the uterus grows during the pregnancy into the third trimester, it usually pulls the placenta up and away from the cervix. This means that most low-lying placentas seen on second trimester ultrasound usually resolve by the third trimester without complications and without needing intervention.

Transvaginal ultrasound is often used to get a better look at the placenta, if there are concerns it may be low-lying.

Your doctor may choose to review the position of the placenta again in the third trimester (around 32-34 weeks) if there are concerns that the placenta remains low-lying.

  • The amount of amniotic fluid around your baby. The volume of amniotic fluid on the second trimester scan is usually subjectively assessed, rather than precisely measured (as happens on a third trimester ultrasound, with the amniotic fluid index).
  • The length of the cervix. This is especially important if you have a history of premature labour, vaginal bleeding or pain. The length of the cervix is not as important as you get closer to your due date (full term). Sometimes a transvaginal ultrasound may be required to get a better view of the cervix.
  • The uterus is checked for conditions such as uterine fibroids (benign muscle growths of the uterus). If fibroids are present, their size and location will be noted. Most fibroids in pregnancy do not cause problems.
  • 3D/4D images of your baby. Many parents enjoy the bonding experienced when they see their baby on 3D/4D imaging. We endeavour to obtain good 3D/4D pictures of the baby’s face on this second trimester scan, but we also think it is important to make sure that your baby is otherwise healthy and growing well. For this reason, the 3D/4D images of your baby are often done after the baby has been fully examined when all routine measurements and other images are completed.

The best time to obtain 3D/4D images of baby is between 24 and 34 weeks.

Ultrasound can assess your baby’s development with great detail, but it cannot detect all problems or abnormalities.

Having your ultrasound in a practice with high standards of expertise and experience will improve the chance of detecting abnormalities. At Advanced Women’s Imaging, we only perform ultrasounds for pregnant women and women’s gynaecologic problems, so we are very experienced in this area.

Unfortunately a normal ultrasound does not guarantee that your baby will be normal nor does it guarantee that you or baby will not develop complications during your pregnancy.

Routine ultrasound is one of the best methods we have of checking your baby and helping detect if you are at increased risk of certain pregnancy complications. A normal ultrasound is reassuring for parents and doctors, but it is not perfect.

Problems such as cerebral palsy, learning difficulties or autism are currently not able to be detected on ultrasound, as they are not associated with structural changes in the baby.

We understand that pregnancy is an exciting time and know that many families are keen to find out the sex of their baby during the pregnancy. The gender of the baby can usually be determined at the 18-20 week morphology ultrasound.

It is good for parents to keep in mind that the pregnancy ultrasound is primarily a medical examination, assessing your baby for significant problems that can sometimes occur. Establishing the gender of your baby is important to us, but you should remember that your sonographer also has many other important things to examine to ensure that your baby is healthy and developing normally.

It is not always possible to establish your baby’s gender with certainty. The position of your baby as well as other factors may hinder the ultrasound’s view of this area of the baby.

If you want to know the sex of your baby, please tell your sonographer at the beginning of the examination. This will give us multiple opportunities to establish the gender of your baby.

If you do not want to know the sex of your baby, please also tell your sonographer at the beginning of the ultrasound. The sonographer will then not focus on this area during the scan.

Most babies (more than 95%) will have completely normal ultrasounds.

Sometimes a problem or change may be detected in your baby. These generally fit into the one of the following categories:

  • Variation of normal. Not all babies look exactly the same on ultrasound. We may see small differences in your baby that occur in some, but not all, babies. We call these differences “variations of normal”. This group includes:
    • echogenic intra-cardiac focus – a small white spot in the inner muscle of the baby’s heart, which does not affect the function of the heart.
    • choroid plexus cysts – small cysts within the choroid plexus area of the baby’s brain, which does not affect the function of the brain.
    • single umbilical artery – most umbilical cords have 2 arteries and one vein, but some babies have only one artery and one vein.

If this is the only difference noted in your baby, then often no further follow-up will be needed.

Sometimes another ultrasound later in the pregnancy may be suggested, such as with single umbilical artery as this is sometimes associated with small babies.

Some of these differences are also known as “soft markers”. This means that the different appearance usually occurs in normal babies, but is known to occur a little more frequently in babies with certain chromosome abnormalities, such as trisomy 21 (Down syndrome). Soft markers for trisomy 21 include echogenic intra-cardiac focus, shortened femur and pyelectasis (dilation of the collecting system of the kidney). If the doctor feels your risk for a chromosome abnormality has increased significantly because of the presence of a soft marker, further testing (such as NIPT or amniocentesis) may be suggested. Most women who have a soft marker seen on ultrasound will not require amniocentesis, because their risk for Down syndrome remains low.

  • Less significant abnormality. A few percent of babies will have less significant, but nevertheless important, abnormalities detected. This group of abnormalities are not considered life-threatening. Some of these problems may require surgery in the neonatal period (such as cleft lip) or other intervention like physiotherapy (such as clubfoot or talipes). Some of these problems may require follow-up ultrasounds during pregnancy, or after delivery (such as dilation of the collecting system of the kidney, which may indicate the presence of urinary reflux).
  • Significant, severe or life threatening abnormality. A few percent of babies will have a major abnormality, such as heart defects or spina bifida, detected on this ultrasound. This group of abnormalities can be associated with pregnancy loss, neonatal death or significant developmental problems. Some of these abnormalities may be corrected with surgery in the neonatal period, with improved outcomes.

We know that finding any change, difference or abnormality on an ultrasound causes a great deal of anxiety in parents. At Advanced Women’s Imaging we deal with these issues regularly. There is always an opportunity to discuss the findings with our ultrasound doctor. We will provide information and arrange further management if required (such as amniocentesis or referral to other specialists).