The blood test prescribed at the very beginning of pregnancy makes it possible to check a large number of biological parameters, in particular the complete blood group of the mother-to-be. This is important information because in the event of negative rh, it will be necessary to take precautions to avoid any complications.

The blood group has two data: letters (A, B, O or AB) and a sign (positive or negative). Biologists determine this data by examining the surface of red blood cells. If they observe the presence of Rh antigen, the person is said to be rh positive. This is the case for 85% of the population. In the absence of this Rh antigen, the person is said to be rh negative. This scenario, which concerns 15% of the population, can be problematic during pregnancy.

Blood groups are passed on hereditarily. Thus, a rh negative woman having a child with a rh positive man can carry a rh positive baby. We then speak of rh incompatibility or foeto-maternal incompatibility.

If this is a first pregnancy, this is usually not a problem: the placental barrier plays its role and the red blood cells of the rhesus positive fetus do not come into contact with the rhesus negative red blood cells of its mother. But during childbirth, when the placenta ruptures to allow expulsion, the baby’s blood cells mix with those of the mother. These latter being carriers of the Rh antigen which it itself lacks, the mother’s organism considers them as foreign cells and instantly produces antibodies called “irregular agglutinins”, or anti-réhsus, to protect themselves from them. . This phenomenon does not affect the newborn baby. But he may have them on the occasion of a future pregnancy.

When a rhesus negative woman has developed “anti-rhesus” antibodies (irregular agglutinins) during a first childbirth and is pregnant again with a rhesus positive fetus, her self-protective antibodies will consider blood cells. red of the latter as intruders. They will then cross the placental barrier to destroy them, exposing the fetus to the risk of severe anemia, jaundice, edema. Even, in the most serious cases, brain damage.

Fortunately, complications related to rhesus incompatibility have become rare, with any rhesus negative expectant mother benefiting from close monitoring. A monthly blood test ensures the absence of irregular agglutinins in his body. If these “anti-rhesus” antibodies harmful to the fetus are detected, the mother-to-be is referred to a specialized center for optimal care. When the anemia of the fetus is very severe, doctors may need to give in utero transfusions through the umbilical cord vein every ten days.

If close monitoring makes it possible to avoid complications linked to rhesus incompatibility, doctors still prefer to act upstream and prevent the development of these famous “anti-rhesus” antibodies by injecting anti-D gammaglobulins. This “vaccination” is routine for rh negative women within 72 hours of their first childbirth. But also in other circumstances that have made it possible for the red blood cells of the fetus to pass into the maternal blood: after an abortion, an amniocentesis, an ectopic pregnancy, a miscarriage. Or when abdominal trauma or hemorrhage, however small, occurs during pregnancy.