If a mother has a clinically significant antibody, fetal blood for phenotyping can be obtained by several procedures, depending on gestational age and the antigen involved. These include
- Amniotic fluid sampling*
- Chorionic villus sampling*
- Cell-free fetal DNA in maternal plasma*
- Percutaneous umbilical cord blood sampling (PUBS) / cordocentesis**
As noted, typing the fetus is warranted when the father's blood type is unknown or the father tests as heterozygous positive.