Prenatal Blood Tests: Complete Pregnancy Testing Guide

Why Prenatal Blood Tests Matter

Prenatal blood tests help monitor your health and your baby's development, screening for conditions that could affect the pregnancy if untreated.

First Trimester Tests

Second Trimester Tests

Third Trimester Tests

Frequently Asked Questions

How antenatal testing is scheduled

Antenatal tests follow a broadly standard timetable. At the booking appointment (ideally before 10 weeks) blood is taken for blood group and rhesus status, a full blood count to check for anaemia, and screening for infections including HIV, hepatitis B and syphilis. Screening for Down's, Edwards' and Patau's syndromes is discussed and, if accepted, performed between 11 and 14 weeks.

Later in pregnancy, a repeat full blood count and antibody screen is usually taken around 28 weeks, along with an oral glucose tolerance test for those at risk of gestational diabetes. Anti-D is offered at this point to women who are rhesus negative. Every test is optional, and your midwife should explain what each one looks for before you decide.

References

Sources cited on this page. PubMed links open the original abstract.

1. ACOG Committee on Practice Bulletins. Prenatal diagnostic testing for genetic disorders. Obstet Gynecol. 2016;127(5):e108–e122. PMID 27607922 · doi:10.1097/AOG.0000000000001404

First trimester screening – the combined test explained

Between 11 and 14 weeks, pregnant women are offered a combined screening test for Down's syndrome (trisomy 21), Edwards' syndrome (trisomy 18), and Patau's syndrome (trisomy 13). It combines:

• Nuchal translucency ultrasound: Measures the fluid-filled space at the back of the baby's neck. A thicker nuchal translucency (above the 95th centile for gestation) is associated with chromosomal abnormalities and some structural heart defects.
• PAPP-A (pregnancy-associated plasma protein A): A blood protein produced by the placenta. Low PAPP-A at 10–14 weeks is associated with chromosomal abnormalities and also with placental problems later in pregnancy (pre-eclampsia, fetal growth restriction).
• Free beta-hCG (human chorionic gonadotrophin): The pregnancy hormone. High free beta-hCG combined with low PAPP-A raises the probability of trisomy 21.

These measurements are combined with maternal age, ethnicity, and exact gestational age to calculate a probability score. A result of 1 in 150 or above is considered "higher chance" in England and triggers an offer of diagnostic testing (NIPT or invasive testing).

NIPT (non-invasive prenatal testing) – cell-free DNA

Non-invasive prenatal testing analyses cell-free fetal DNA that circulates in the mother's bloodstream from approximately 9–10 weeks of pregnancy. Cell-free fetal DNA (cfDNA) – fragments of placental DNA – makes up approximately 10–15% of total cfDNA in maternal blood. NIPT for the common trisomies has a detection rate above 99% for trisomy 21, with a false positive rate below 0.1% – far more accurate than the combined first trimester screening test.

NIPT does not diagnose a chromosomal condition – it calculates a revised probability. A high-chance NIPT result should be confirmed by amniocentesis or chorionic villus sampling (CVS) before any irreversible decision is made. The NHS began routinely offering NIPT to women with a higher-chance combined test result in 2021; private NIPT is available from 10 weeks for women who prefer earlier reassurance.

Second trimester screening – the anomaly scan and AFP

The mid-pregnancy anomaly ultrasound scan (18–21 weeks) is the primary screen for structural abnormalities: heart defects, neural tube defects (spina bifida, anencephaly), abdominal wall defects, and limb abnormalities. It detects approximately 50% of major structural abnormalities and has a near-100% detection rate for anencephaly. If neural tube defect is suspected clinically or by elevated maternal serum alpha-fetoprotein (AFP), the anomaly scan is the principal confirmatory investigation. AFP is also elevated in abdominal wall defects (gastroschisis, omphalocele) and multiple pregnancy.

Group B Streptococcus (GBS) screening is not offered universally in the UK (though it is in the USA), but vaginal and rectal swabs for GBS may be taken around 35–37 weeks in women at high risk of neonatal GBS infection, including those with a previous affected baby or preterm labour.