About Prenatal Ultrasound
Most couples want to obtain an ultrasound during the pregnancy, and it is estimated that 70-80 percent of pregnant women do have an ultrasound in the United States. This can be a reassuring event that the baby looks normal and can be fun watching the baby move, as well as get some pictures (we offer 3D) or a short video to take home.
What can you expect from an ultrasound at the various stages of pregnancy?
One expectation is for us to date a pregnancy in weeks from the beginning of the last menstrual period. All pregnancies are calculated clinically from the beginning of your last menstrual period even thought you were not pregnant then.
We can first see signs of a pregnancy at about 5 - 5.5 weeks, when, on transvaginal ultrasound, we will see a gestational sac in the uterus about 1-3 millimeters in diameter.
Sometimes we cannot even see a sac but can demonstrate blood flow using Doppler that indicates a pregnancy has occurred or is occurring in the uterus. If we don't see a sac, we are careful because things change daily at this stage of pregnancy. A day or two later we may see the sac or yolk sac, and a day or two later the embryo. As soon as we can see the embryo we may see the pulsation of the embryonic heartbeat.
At this early stage, we can sometimes diagnose problems: 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. Your doctor may want to do serial ultrasounds to determine if normal development is taking place.
Around 7 and 8 weeks the embryo is very well seen, and we should begin to see limbs. The head, abdomen, umbilical cord and its insertion into the embryo's body, as well as the location of the placenta are clearly visualized.
Measuring the length of the embryo (crown-rump length) at this stage of pregnancy gives us very accurate dating of the pregnancy, usually within 3 or 4 days. The further along in pregnancy the first dating is done, the less accurate it is.
If your dates are uncertain (and there are lots of reasons you might not be sure how far along you are) then the first ultrasound you have sets the due date, and that is not changed even with subsequent ultrasounds.
For example, if your first ultrasound, done at 8 or 10 weeks, says you are due on the 4th of July, and an ultrasound done 6 or 8 weeks later says you are due the 20th of October, the second ultrasound does not change the due date. The second ultrasound should be interpreted that the fetus is not growing at the expected rate (the fetus is not as large as it should be).
The Late First Trimester
At about twelve weeks gestation, the embryo changes its 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. We can see a lot of detail with transvaginal ultrasound at 11 or 12 weeks if we take the time to look.
This is the time that we want to look at the nuchal translucency.
The Second Trimester
The ideal time to do an ultrasound during a pregnancy is between 16 to 22 weeks.
At this point the dating accuracy is reasonable, and the fetus is large enough to do a detail survey of the fetal anatomy. As the fetus becomes larger, transvaginal or translabial scanning may be useful to look at the cervix or to look at the part of the fetus that is near the cervix.
Amniotic fluid which surrounds the fetus provides an excellent window for allowing us to evaluate the fetus and is usually plentiful at this gestational age. The fluid assists in allowing the sound waves to pass with strength and clarity. Also the fetus has the room to move and roll, and if we are lucky, the fetus 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 fetus so the sound waves do not travel as well. Also the fetus has less room to move, so that if the fetus is in an unfavorable position for looking at a particular part of the body, the fetus is likely to stay in that 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 fetus is growing.
If the first scan was done exactly twelve weeks ago, but the baby in that time grew 8 weeks worth, then we know the fetus has not grown as it should.
If, in exactly twelve weeks from the last scan, the fetus has grown 14 weeks worth, then we know we are dealing with a large fetus.
Third Trimester Scans
Most abnormalities of the fetus 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, as well as to determine the position of the fetus. 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 fetus does not roll around much. The fetus anatomy is larger, and therefore those parts of the fetus that are in a favorable position can be seen well, whereas those parts that are in an unfavorable position may not be seen well at all.
Prenatal ultrasound cannot diagnose all problems with a fetus, so you should never interpret a normal prenatal ultrasound exam as a guarantee that you are going to have a normal baby.
Some abnormalities are very difficult to find, or may not have developed at the time in the pregnancy that you have your scan. The position of the fetus in the uterus has a great deal to do with how well we see structures. You may be asked to return in an hour or two or even a day or two to complete the examination, in the hopes the fetus has moved.
Images tend to be dramatically clear in thin patients with lots of amniotic fluid and not so clear in heavy women, particularly if there is not much amniotic fluid.
Excerpt from the Medical Ultrasound Fact Sheet from the American Institute of Ultrasound in Medicine (AIUM)