Obstetrical Ultrasound Imaging

Ultrasound (US) imaging uses an ultrasound machine to obtain pictures of a pregnant woman and her embryo or fetus.

An ultrasound machine sends high frequency sound waves into the body. These waves reflect off of tissues and are recorded and displayed as a picture. US does not use any radiation or x-ray.

What is Obstetrical Ultrasound used for?

Most couples want to obtain an ultrasound during a pregnancy, and it is estimated that 80 percent of pregnant women in this country have an ultrasound at some point during their pregnancy. It is fun to see the baby, obtain some photos, and be reassured that the baby looks ok. In a normal pregnancy, however, please understand that an ultrasound is not required. Below is a brief description of an obstetrical ultrasound performed at different times during a normal pregnancy.

How to prepare for an Obstetrical Ultrasound.

Please wear a loose fitting, two-piece outfit for the exam. Only the lower abdomen needs to be exposed during this exam. With a two-piece outfit you will not have to remove your clothes. Please do not bring young children, if possible, as they may disrupt the exam. If you are in the first trimester, you do not need to have a full bladder. If you are in the second or third trimester, you do need to have a full bladder.

Early First Trimester

We can first see signs of a pregnancy at about 5.5 weeks, when transvaginal ultrasound shows a gestational sac in the uterus about ¼ inch long. Things change daily at this stage of pregnancy. A day or two later the yolk sac appears, and a day or two later the embryo. With the embryo, we see the pulsation of the embryonic heartbeat.

Even at this early stage, we can sometimes diagnose a problem. If the embryo's heart rate is very slow or the yolk sac is excessively large, that might be a sign the pregnancy is going to fail. If the embryo or heart beat are not seen at the expected time in pregnancy, your caregiver may check to be sure the pregnancy dating is correct.

By 8 weeks the embryo is well seen. Measuring the length of the embryo, called the “crown-rump length,” at this time gives the expected due-date very accurately, to within 3 or 4 days. The further along in pregnancy the first dating is done, the less accurate it is.

An important point to make here: if your dates are uncertain (and there are lots of reasons you might not be sure how far along your pregnancy is) then the first ultrasound you have effectively establishes the due date, and that is not changed by subsequent ultrasounds. For example, if your first ultrasound, done at 8 or 10 weeks, indicates that you are due on the 1st of May, and an ultrasound done 6 or 8 weeks later suggests that you are due the 20th of May, the second ultrasound does not change the due date. The second ultrasound should be interpreted that the baby is not growing at the expected rate (the baby is not as large as it should be).

The Late First Trimester

At 10 weeks the fetus starts to resemble a baby. We can see arms and legs, hands and feet, head and brain, the developing fetal spine, and sometimes other organs. The fetus moves and jumps about. It is amazing how much we can see with transvaginal ultrasound at 11 or 12 weeks if we take the time to look. This is the time that we want to obtain a nuchal translucency measurement, which is one way to partially screen for chromosomal abnormalities.

The Second Trimester

If you have only one ultrasound during pregnancy, it should be done halfway through the pregnancy, about 20 weeks. At this point the dating accuracy is reasonable (plus or minus about two weeks). This is an excellent time to look at fetal anatomy, because the fetus is developed enough to permit visualization of most of the important fetal organs. There is also normally a sufficient amount of fluid surrounding the fetus at this point in the pregnancy, which further improves our ability to evaluate the fetus sonographically. Later in pregnancy there is less fluid in relation to the size of the baby so the sound waves do not travel as well. Also the baby has less room to move, so that if the baby is in an unfavorable position for looking at a specific part of the body, the baby is likely to remain in that unfavorable position.

If an ultrasound was done early in pregnancy (and we have the results of that first scan) then a second trimester ultrasound not only permits us to look for abnormalities, but also to evaluate how the baby is growing. If the first scan was done exactly 10 weeks ago, but the baby in that time has grown only 7 weeks worth, then we know the baby has not grown, as it should. If, in exactly 10 weeks from the last scan, the baby has grown 12 weeks worth, then we suspect you have a large baby.

Third Trimester Scans

Many abnormalities of the baby can be detected in the second trimester, but there are some abnormalities that do not develop until later. Most third trimester ultrasound scans are ordered to determine if growth is normal, to try and estimate the weight of the fetus, and occasionally to determine the position of the baby. Third trimester scans also are ordered to follow up on possible abnormalities seen at an earlier scan. As mentioned previously, in the third trimester there is less amniotic fluid, and the baby does not roll around much. The baby and its anatomy is larger, and therefore those parts of the baby that are in a favorable position can be seen very well, but those parts that are in an unfavorable position may not be seen well at all.

Other Items

Gender can usually be determined by 20 weeks, BUT ultrasound is not always accurate.
There is a large difference in the quality of the images we obtain from one patient to another. Thin women tend to have good images and heavy women tend to have poor images, but it is not always that way.
Vaginal scanning gives excellent images but we can only examine what is very near the end of the transducer, and of course we cannot move the transducer around very much.
In our Country there is no standard of medical care that says every pregnant woman should obtain an ultrasound exam during pregnancy. In some European countries there are laws that require one or two ultrasound exams in every pregnancy. It has not been possible to show conclusively that routine prenatal scanning (as contrasted to obstetrically indicated scanning) improves obstetrical outcome.
Estimating fetal weight by ultrasound is not always precisely accurate.
Prenatal ultrasound is not perfect. Ultrasound has its limitations; it can and does miss things. One can never guarantee a normal baby.

Obstetrical ultrasound may be ordered for several reasons. These include:

To check if a woman is pregnant.
Estimate the age of the fetus.
To evaluate if the fetus is growing correctly.
To evaluate the position of the fetus within the uterus.
To identify the position of the placenta, which is the tissue that transfers oxygen from the mother to the fetus.
To assess for the presence of absence of multiple gestations (twins).

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