Clinical Biochemistry - First Trimester Combined Screening

The department provides First Trimester Combined Screening for Down’s (trisomy 21), Edwards' (trisomy 18) and Patau's (trisomy 13) syndromes for Southmead, St Michael’s and Weston General Hospitals and for the Bath area. 

First Trimester Combined Screening involves ultrasound measurement of nuchal translucency (NT) and laboratory measurement of maternal serum PAPP-A and free beta-HCG.  A statistical risk calculation is performed to estimate the chance of Down’s syndrome and a combined chance of Edward's and Patau's syndromes.

First Trimester Combined Screening is the test recommended by the UK National Screening Committee and may be performed within the gestational age range of 11 weeks + 2 days to 14 weeks + 1 day.

Second trimester Down’s syndrome screening, based on the quadruple test, is also available for women booking too late for first trimester screening.  The quadruple test may be performed within the gestational age range of 14 weeks + 2 days to 20 weeks + 0 days (ideally at 15 to 16 weeks gestation). Samples are referred to the Royal Victoria Hospital, Newcastle.

A 20 week anomaly scan is used to screen for Edwards' and Patau's syndromes in the second trimester.

The First Trimester Combined Screening service has been developed by close collaborative working with maternity services of three Trusts and conforms to the standards and recommendations set by the National Screening Committee.

For more information see