The First Trimester Ultrasound
Many women have a dating or viability scan in the first trimester. In women who are unsure of their last menstrual period or have irregular periods, the dating scan can help to more accurately determine the age of the baby and the expected due date. The scan can also note the presence of multiple babies (twins, triplets), and the presence of a heartbeat. Because of the small size of the baby at this point, this ultrasound may be performed with a transvaginal transducer, or a small probe that enters the vagina to enable better pictures of the uterus.
Women who experience first trimester bleeding or spotting are often evaluated by ultrasound. First trimester bleeding is common, affecting 20-30% of all pregnancies. Ultrasound can help to identify the cause of bleeding, which can include a small subchorionic hemorrhage, or collection of blood between the developing baby and wall of the uterus. Ultrasound can also detect an ectopic gestation, or a pregnancy outside of the uterus, a rare but serious cause of bleeding. Ultrasound can also assess for signs of an impending miscarriage, including an absent fetal heartbeat and an open cervix.
Late in the first trimester or early in the second trimester (between 11 and 14 weeks), some women undergo a special ultrasound called a nuchal translucency screening or nuchal scan. This test measures the amount of fluid accumulated behind the baby’s developing neck; an increased amount of fluid can be associated with Down syndrome and other genetic disorders.
The Second Trimester Ultrasound
Most pregnant women in the US have an ultrasound at 20 weeks, also known as the ‘anatomy scan’. This is a detailed examination looking for abnormalities in the baby’s brain, face, spine, heart, stomach, kidneys and other structures. The overall size of the baby and its position in the uterus are also assessed. Any abnormalities that might make a vaginal delivery unsafe, such as a breech (feet-first) position or abnormal placement of the placenta, are noted. Structural abnormalities such as a cleft palate or heart malformation may require further, more detailed, ultrasounds, and may indicate that the baby could be more safely delivered and cared for in a specialized hospital setting.
The Third Trimester Ultrasound
Third trimester ultrasounds are frequently ordered to follow-up abnormalities seen on previous scans. For example, breech babies often turn into the proper head-first position on their own. These scans give doctors and midwives the information they need to plan a safe delivery.
Ultrasounds may also be performed late in the pregnancy to gauge the health of the baby; a biophysical profile (BPP) assesses the level of fluid around the baby, as well as the baby’s movements and muscle tone. This can help doctors decide if the baby is under stress, and may need to be delivered early.
One final note: in the past decade, a number of nonmedical commercial ultrasound centers have opened, offering three- and four-dimensional ‘keepsake’ ultrasound images and videos. In general, organizations including the FDA and American Institute of Ultrasound in Medicine (AIUM) discourage the use of these facilities, because the staff may not be as well trained as medical sonographers, and they may utilize ultrasound at higher levels and for longer exposure times than a typical medical scan. A picture is worth a thousand words, but the real thing is worth waiting for.