How is fetal RhD status determined antenatally, and what maternal serology is done?

Non-invasive prenatal testing (NIPT) detects cell-free fetal DNA in maternal blood and uses PCR of the RhD gene to determine the fetal RhD status without invasive sampling.

Maternal serology: blood group and Rh status at booking; if RhD-negative, check for unexpected antibodies using the indirect antiglobulin (indirect Coombs) test, which detects and quantifies maternal antibodies that can cross the placenta.

Clinical relevance. NIPT and indirect Coombs testing identify at-risk fetuses and the degree of sensitisation, guiding surveillance.

More Detail

Determining fetal RhD status and maternal serology

Antenatal testing identifies at-risk fetuses and the level of maternal sensitisation.

Fetal RhD status

• Non-invasive prenatal testing (NIPT) - cell-free fetal DNA in maternal blood is analysed by PCR for the RhD gene sequence to determine the fetal RhD status.
• This is safe (no procedure-related risk) and especially useful when the father is heterozygous.

Maternal serology

• Blood group and Rh status at the booking visit.
• If RhD-negative, screen for unexpected (atypical) antibodies using the indirect antiglobulin test (IAT) / indirect Coombs test on untreated maternal red cells - detecting antibodies that could cross the placenta.
• If antibodies are found, the indirect Coombs titre quantifies the degree of alloimmunisation, guiding surveillance (e.g. serial titres and MCA Doppler).

Note

A sensitised mother has a positive indirect Coombs test; the fetus/newborn would have a positive direct Coombs test.