First trimester ultrasound is usually performed between 11-14 weeks.
This ultrasound is important for early identification of fetal abnormalities.
When used with a blood test called PAPP A and Free Beta HCG it can also provide information about placental function, preeclampsia (high blood pressure developing in pregnancy) risk and provide an estimate of risk of having a baby with Down syndrome, or Trisomy 18/13.
Because it also evaluates the thickness of the fetal neck it also provides information about risks of the baby having a congenital heart defect.
Some patient are having early testing with a blood test for Down syndrome called ccf DNA or NIPT and are foregoing the first trimester ultrasound.
The lack of ultrasound evaluation at 12 weeks could result in an unexpected surprise at the 20 week ultrasound.
Combined evaluation with blood testing and early ultrasound is the most comprehensive evaluation available.