What is the best gestational age for first trimester tests?

The median nuchal translucency, PAPP-A and hCG MoM levels change with gestational age during the first trimester.

The screening performance for NT is greater at 11 weeks than at 12 or 13 weeks, for PAPP-A it is better at 10 weeks and for hCG at 12 - 13 weeks. The best time to take the measurements is therefore a compromise and it is reasonable to target 11 weeks as the best time for obtaining first trimester measurements (see references 1 and 2).

References

1. First and second trimester antenatal screening for Down’s syndrome: the results of the Serum, Urine and Ultrasound Screening Study (SURUSS) (2003)
2. SURUSS in perspective (2004)