First trimester screening is a combined test that estimates the risk of a fetus having certain chromosomal abnormalities, particularly Down syndrome (trisomy 21) and trisomy 18, using a combination of ultrasound and maternal blood work. The ultrasound component measures the nuchal translucency, a fluid-filled space at the back of the fetal neck, between 11 and 14 weeks of gestation. This measurement is combined with maternal blood levels of pregnancy-associated plasma protein A and free beta-human chorionic gonadotropin, along with maternal age, to calculate a risk estimate. Additional ultrasound markers such as the nasal bone and ductus venosus waveform may also be incorporated to improve the screening accuracy.
Sonography Term