A BRIEF OVERVIEW OF PRENATAL ULTRASOUNDMost couples want to obtain an ultrasound during a pregnancy, and it is estimated that 80 percent of pregnant women do have an ultrasound in this Country. It is fun to see the baby, obtain some photos or video, and be reassured that the baby looks ok.
I would like to talk about what to expect from ultrasound at the various stages of pregnancy. Remember that we date a pregnancy in weeks from the beginning of the last menstrual period. We know you were not pregnant at the beginning of the last menstrual period, but that is the way we do it.
EARLY FIRST TRIMESTER
We can first see signs of a pregnancy at about 5.5 weeks, when, on transvaginal ultrasound, we will see a gestational sac in the uterus about 2 or 3 millimeters in diameter. Things change daily at this stage of pregnancy. A day or two later we see the yolk sac, and a day or two later the embryo. As soon as we can see the embryo we see the pulsation of the embryonic heartbeat.
At this early stage, we can sometimes diagnose a problem: if the yolk sac is excessively large, that might be a sign the pregnancy is going to fail. If we do not see an embryo or heart beat at a time in pregnancy when we should see that, we need to investigate to be sure the pregnancy dating is correct. We will want to do serial ultrasounds every few days to determine if normal development is taking place.
Around 7 and 8 weeks the embryo is very well seen, and we can try to see four limbs, the head, the umbilical cord and its insertion into the embryo's body, and the location of the placenta. Measuring the length of the embryo (we call it a crown-rump length) at this stage of pregnancy gives us very accurate dating of the pregnancy, plus or minus 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 sets the due date, and that is not changed by subsequent ultrasounds. For example, if your first ultrasound, done at 8 or 10 weeks, says you are due on the 1st of May, and an ultrasound done 6 or 8 weeks later says 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 ten menstrual weeks we change the name from embryo to fetus. The fetus looks like a baby now. We can see arms and legs, hands and feet, head and brain, 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 do the nuchal translucency.
THE SECOND TRIMESTER
If we are given only one chance to do an ultrasound during a pregnancy, it should be done about 18 to 22 weeks. At this point the dating accuracy is reasonable, and the fetus is large enough that we can do a survey of the fetal anatomy by transabdominal scan. As the fetus becomes larger, transvaginal scan becomes less useful, except to look at the cervix and look carefully at that part of the fetus that is near the cervix.
Another reason that this is a good time to do a scan: there is a generous amount of amniotic fluid at this time, and fluid assists in allowing the sound waves to pass with strength and clarity. Also the baby has the room to move and roll, and if we are lucky, the baby will move in just the manner that lets us see better. 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 part of the body, the baby is likely to stay 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 ten weeks ago, but the baby in that time grew 7 weeks worth, then we know the baby has not grown as it should. If, in exactly ten weeks from the last scan, the baby has grown 12 weeks worth, then we know we are dealing with a large baby.
THIRD TRIMESTER SCANS
Most 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. Also third trimester scans 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, and those parts that are in an unfavorable position may not be seen well at all.
OTHER ITEMS
- Gender can usually be determined by 18 weeks, but we are 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 highly accurate.
- Prenatal ultrasound is not perfect. We can and do miss things. We can never guarantee a normal baby.
Joseph A Worrall MD RDMS
This page last updated
April 16 2005