Blood clotting or coagulability and the opposite reaction, fibrinolysis, are in a subtle balance in the blood and can be shown to be present only at the end of the embryonic period.
(platelets) play a central role in the clotting of blood. They are formed through the disintegration of megakaryocytes that can be found in the liver and spleen from the 12th week of pregnancy onwards. Later, the bone marrow takes over the production of megacaryocytes. After the middle of the pregnancy, the level of thrombocytes amounts to 250'000/mm3, which corresponds to an average normal value for adults. Up to birth, though, the thrombocytes remain still very immature in their functioning.
With clotting factors one distinguishes between vitamin K dependent and vitamin K independent factors.
The fetal level of vitamin K independent clotting factors lies at ca. 50% that of the adult normal value.
That of the vitamin K dependent ones can be determined following the end of the embryonic period, just with as the previous ones, but it comes to only 10-30% of the adult normal value. This is mainly due to the fact that at birth the vitamin K reserves are too low and, on the other hand, that the hepatocytes are not yet mature enough to synthesize the vitamin K dependent factors in sufficient amounts. Maternal milk also contains very little vitamin K. For this reason every newborn must be given 1 mg vitamin K after birth in order to safeguard against bleeding in the gastrointestinal tract or in the central nervous system, respectively.