Early fetal sexing from maternal blood sample

Objectives

SE SCOTLAND REGIONAL MOLECULAR GENETICS SERVICE - Cell‐Free Fetal DNA Sex Determination

Introduction

Free fetal DNA may be detected in maternal plasma from early in gestation and used for the determination of fetal gender. The sex of the fetus is determined by the presence of Y‐specific sequence (SRY/ZFY) in the cell free DNA extract for a male fetus and the absence of this in the case of a female fetus. Maternal samples can be tested from 9 weeks gestation (confirmed by scan).

The analytical sensitivity and specificity of the real‐time quantitative PCR assay was measured in 70 pregnancies over a period of approximately 1 year from January to November 2012. When audited against QF‐PCR and karyotype analysis of fetal sex from invasive testing, the test demonstrated 100% concordance with no false positives or false negatives. In due course these results will be audited against pregnancy outcomes.

Service offered

We offer this service to pregnancies at risk of X‐linked disorders, to guide invasive testing and pregnancy management. It is not available for non‐medical indications. This test may not be applicable in multiple pregnancies including those with a possible vanishing twin.

A male fetus is detected by the presence of SRY and ZFY‐specific sequence. Absence of SRY and ZFY in the presence of a control marker and a fetal specific marker is taken as evidence that the fetus is female.

Please note that this test will not detect sex chromosome abnormalities such as Turner syndrome (XO) or Klinefelter syndrome (XXY).

Referrals

All referrals should be made in discussion with the local Clinical Genetics Department. Please contact the laboratory in advance of sending a sample to facilitate rapid processing upon receipt. Samples are accepted from patients from 9 weeks gestation (confirmed by scan) at which time there should be a sufficient concentration of free fetal DNA in the circulation. If the sample will be received within 24 hours of venepuncture, 2 x 7.5ml EDTA blood tubes can be sent. If the sample will take longer than 24 hours to arrive, 2 x 7.5 ml Streck Cell‐free DNA tubes should be used (available from the laboratory upon request).

Information on the outcomes of the pregnancy will be requested for local audit purposes and information sheets for parents about the sexing assay are available on our laboratory website or by request.

Technical

Plasma is isolated from maternal blood as rapidly as possible after collection. Cell free DNA is then extracted from the plasma. Molecular analysis is performed using real‐time quantitative PCR and Taqman assays for the Y specific markers SRY and ZFY, the fetal specific marker RASSF1A and a Beta‐actin and TERT or RNase P control marker.

Target reporting time

The results of the Y‐specific probe analysis should be available within 3 working days of the laboratory receiving the sample.