What does a fetal ultrasound show?
Fetal ultrasound measurements can show how the baby is growing and detect abnormalities. During pregnancy, many different ultrasounds measurements can be done. Fetal ultrasound measurements can include the crown-rump length (CRL), biparietal diameter (BPD), femur length (FL), head circumference (HC), occipitofrontal diameter (OFD), abdominal circumference (AC), and humerus length (HL), as well as calculation of the estimated fetal weight (EFW).
Why is a fetal abdominal circumference important?
The fetal abdominal circumference is the single most important measurement to make in late pregnancy. It reflects a more clear picture of the fetal size and weight rather than age. Serial measurements are useful in monitoring growth of the fetus. Abdominal circumference measurements should not be used for dating a fetus, but rather for estimating the size and weight of the fetus.
Why does the doctor measure my stomach during prenatal appointments?
It's not just your baby's abdominal size that gives vital information; the size of your own abdomen can also provide information on the pregnancy and your baby.
Prenatal visits, in the beginning, are rather uninteresting. There is no heartbeat for the first 10 weeks or so and you don't have an ultrasound to look forward to until much later in the pregnancy. All you have to do is show up once a month, pee in a cup and step on the scale of horror. But, starting in the 20th week of the pregnancy things start getting a little more interesting. The size of your belly starts telling you secrets about baby inside.
What is an abdominal measurement?
The doctor will place a measuring tape at the top of your pubic bone and run the tape to the top of your growing belly. That measurement, in centimeters, should be about the same as your current week of pregnancy. For instance, if you are 24 weeks pregnant you will measure about 24 centimeters.
What causes the measurement to be off?
Babies can grow larger or smaller than expected and the amount of amniotic fluid also comes into play. If mom is overweight or underweight, measurements will be slightly off. In my case, the measurement in my 34th week was 62 centimeters, but I was carrying twins. In that case, the measurement was about right for two single pregnancies.
If you consistently measure much larger than expected and you are of normal weight your doctor may suggest an ultrasound to rule out multiples and measure amniotic fluid. If there is nothing out of the ordinary found and there are no medical concerns to worry about, then you are likely just measuring larger than normal.
Abdominal circumference measurements are adapted from Hadlock FP, et al, AJR 139:367, 1982.