A prenatal ultrasound uses high frequency sound waves to create an image of the uterus, placenta and fetus. An ultrasound can be performed throughout the entire pregnancy, however it is usually performed at specific times, such as in the first trimester (between 11-14 weeks) to obtain the nuchal translucency and accurate dating and in the second trimester (between 18-22 weeks) to survey the fetal anatomy. Depending on the purpose and timing in pregnancy of the ultrasound, it can take anywhere between 20 minutes to 2 hours. Ultrasound can be performed either abdominally or transvaginally and is safe during pregnancy.
3D and 4D Ultrasound
Using similar technique and technology as with the 2D ultrasound, a 3D ultrasound can produce detailed images of the fetus similar to a photograph. This is particularly useful if certain/specific abnormalities are suspected. 4D ultrasound enables real-time 3D imagery of the fetus.
Fetal echocardiography is a specialized ultrasound to evaluate the fetal heart. The images are interpreted by a specialist in congenital heart disease. As with prenatal ultrasound, imaging gel and the probe are applied to the mother’s abdomen. It is not painful and causes no harm to the baby. A fetal echocardiogram may require anywhere from 30 minutes to over 2 hours, depending on the complexity of the baby’s heart. Fetal echocardiograms can reliably be performed any time after 17-18 weeks, however as technology progresses and with the use of transvaginal ultrasound, the ability to obtain images of the fetal heart as early as 13 weeks may be possible.
A fetal MRI (magnetic resonance imaging) is a diagnostic technique. It manipulates magnetic fields to obtain views of the baby, its placenta, and the maternal uterus and body from many different angles. It can be especially useful for obtaining images of the fetal brain, chest, and abdomen. During an MRI, the mother must lay as still as possible on either her back or side. The study usually takes approximately 30-45 minutes. MRI during pregnancy is believed to be safe with no adverse risks on unborn babies. As the machine is a giant magnet, no metal objects are allowed inside of it.
Diagnosis and Screening
Chorionic Villus Sampling (CVS)
CVS is a prenatal test typically performed between 10-14 weeks of pregnancy. It is the withdrawal of a small amount of chorionic tissue (part of the placenta), by either a catheter through the cervix or a needle inserted into the abdomen. The procedure is performed under ultrasound guidance. CVS can detect chromosome abnormalities, such as aneuploidy, rearrangements, and major duplications/deletions, with 99.6% – 99.8% accuracy. The risk of miscarriage associated with the procedure is 1/200 (0.5%).
Amniocentesis is typically performed between 15-23 weeks of pregnancy. In this procedure, a small amount of amniotic fluid (fluid surrounding the unborn baby) is withdrawn from the uterus by a thin needle through the abdomen. Like the CVS, the procedure is done under ultrasound guidance and can detect chromosome abnormalities, such as aneuploidy, large duplications/ deletions, and large rearrangements, with 99.6%-99.8% accuracy. The procedure can also detect open neural tube defects (spina bifida and anencephaly) and abdominal wall defects with approximately 95% accuracy. The risk of miscarriage associated with the procedure is 1/300 (0.3%).