“Ultrasound is the modality of choice when imaging the pregnancy and fetus. It is noninvasive, safe due to absence of radiation, low in cost and has widespread availability. The technique has high accuracy and superior spatial resolution, allowing real time, color Doppler, multiplanar and 3-4 dimensional capabilities.
The first prenatal ultrasound is usually done in the first trimester, with the purpose of confirming the pregnancy as well as to date the pregnancy. In addition, it is able to help establish the location and size of the fetus, as well as the number in the case of multiple gestations. It can be performed as part of the first trimester ultrasound genetic screening, as well as to screen for any anomalies of the uterus or cervix. In the first trimester, use of spectral Doppler is discouraged as it delivers greater energy to the fetus than simple 2-dimensional or M-mode imaging.
The next prenatal ultrasound is routinely done in the second trimester, between 16 and 22 weeks gestation, with the aim of evaluating in greater detail the fetal anatomy and identify any birth defects. In addition, it is possible to evaluate and follow fetal growth, and examine the placenta as well as the quantity of amniotic fluid. In the event that a fetal anomaly is detected, ultrasound may also be used to guide certain diagnostic procedures such as amniocentesis and chorionic villus sampling which may identify the cause.
An additional ultrasound in the third trimester is not standard practice. However, it may be done in the following instances, to name a few: monitoring of fetal growth, assessing evolution of any detected fetal anomaly, or determine fetal position before delivery, as well as to confirm presence of a normal fetal heartbeat if the fetal movements become reduced or absent.
Throughout pregnancy, ultrasound may be used to determine the cause of unexplained vaginal bleeding or other complications. It may also be used to guide certain procedures that are performed in an attempt to improve the outcome of a fetus with a given problem, e.g. amnioreduction/amnioinfusion, antenatal shunt placement, or ablation in the context of Twin-Twin Transfusion Syndrome.
Details on recognized indications of ultrasound for antenatal imaging are available at:
- ACR-ACOG-AIUM Practice Guideline for the Performance of Obstetrical Ultrasound (American College of Radiology)
Fetal ultrasound should be done only for valid and recognized medical indications, and not for creation of “keepsake” videos or pictures, or determination of fetal gender outside of a medical context as is done in certain cultures.” Read More