For most expectant parents, seeing a prenatal ultrasound is a highlight of the pregnancy. New parents often cannot wait to get a glimpse of their unborn child and to find out the sex. Of course, your doctor has her own reasons for ordering this routine test. What she's looking for often depends on how far along your pregnancy is.
An initial ultrasound is often performed to confirm the age of the developing baby and to verify the due date. This is especially important when the mother is unsure about her last menstrual period, or if her periods are irregular. Ultrasound can also help in early pregnancy to find a heartbeat if it cannot be heard with a simple hand-held Doppler. An ultrasound, along with blood tests, can be performed between 11 and 14 weeks to help assess for certain birth defects, such as Down syndrome. The ultrasonographer measures the skin thickness at the back of the fetal neck. Increased thickness can be a sign of Down syndrome or other birth defects.
A routine ultrasound to survey basic anatomy and growth is often done at around 20 weeks. The purpose is to check vital organs, including the head, heart and kidneys. The sex of the baby is usually evident at this ultrasound. Doctors will often gauge your developing baby's growth at this time and later during pregnancy using measurements of the head, abdomen and femur. Many serious birth defects can be discovered at or before this time. It is important to remember that some birth defects may be missed by ultrasound, or may be apparent only at a later date.
Positioning and Number
An ultrasound is useful in letting parents know whether they will be welcoming one baby, twins, triplets -- or more. A doctor can also verify the position of the baby close to term and make sure she is in the correct position, with her head down, and not breach or sideways.
Ultrasound for Placenta, Amniotic Fluid
Ultrasound is also used to check the location of the placenta. A placenta covering the birth canal can cause bleeding during pregnancy and hemorrhage during labor. Doctors often use ultrasound to measure amniotic fluid volume. The amount of amniotic fluid is important when the mother has a medical condition, such as diabetes or high blood pressure, or is past her due date. Amniotic fluid is also measured when the distance from the mother's pubic bone to the top of her uterus, also known as fundal height, is less than or greater than that expected at that stage of pregnancy.
An ultrasound is used as part of a biophysical profile test. This test is performed if the mother is past her due date, is not feeling the fetus move well, or has a medical condition that causes problems with fetal growth or the amniotic fluid. During the ultrasound part of this test, the doctor measures fetal breathing movements, body movements, muscle tone and amount of amniotic fluid.
It has become commonplace for women even with low-risk pregnancies in the U.S. to undergo at least 1 screening ultrasound. This practice has led to a dramatic increase in the cost of prenatal care. Most clinical studies have not found that these routine screening ultrasounds are associated with better outcomes for the baby, as measured by such complications as cerebral hemorrhage, seizures or lengthy stays in the special care nursery. This routine screening also does not appear to be associated with decreased newborn death rates. Ask your doctor if you have any concerns about your pregnancy or if you are unsure why an ultrasound is being performed.