This is often the first scan that is advised in a pregnancy. This scan is done prior to 11 weeks to confirm the number of pregnancies and if it is twins or more. If a twin pregnancy is identified, it is possible to accurately determine whether they are monochorionic (sharing the same placenta) or dichorionic (having separate placentae). Twins sharing same placenta need close monitoring and the first trimester scan report helps the obstetrician in planning further course of management of the pregnancy.
During this scan, a reasonably accurate dating can be done and after 12 weeks, anatomical evaluation of the fetus can be done
During the anatomical evaluation, the fetal head, spine, upper and lower extremities, stomach, urinary bladder and to a certain extent the fetal heart can be visualised . Sometimes to see the baby better, an internal (vaginal) scan may be needed
There are 3 conditions for which we can “screen” during this period :
A. Chromosomal abnormalities – especially Down Syndrome
B. Pre eclampsia
C. Fetal anomalies
All babies have a small amount of fluid behind the neck which is called "Nuchal translucency". The measurement of the thickness of this fluid is recommended during a scan done at this time. This by itself, or in combination with a blood test,helps in predicting the risk for Down syndrome. (see section on FTS ), Trisomy 13 and Trisomy 18
Screening for Pre-eclampsia is done by a combination of Maternal history, Maternal blood pressure, Uterine artery doppler and biochemical parameters and forms aprt of first trimester screening to be corrected as scanning done between 11-14 weeks
There are a number of major abnormalities that can be identified in the first trimester. However it should be emphasised that some anomalies would require a further confirmation either around 16 weeks or at 18-20 weeks. The human embryo undergoes rapid changes with time and some anomalies may evolve with time. About 50% of major anomalies can be identified during this time.
