Approximately 60% of all human pregnancies terminate spontaneously before birth, mostly before being recognized. Exceptionally high prevalence of abnormalities has been found in this reproduction output. Original frequency of chromosomal (ChA) and developmental anomalies (DA) is not known, but may be very high. From 100 spontaneously aborted fetuses about 50 are ChA, approximately 70 are DA and only about 20 are without any pathological finding. Using data on ChA and DA distribution in newborns and those in abortuses--together with the knowledge on abortion rates in various gestation phases--the prenatal population structure may be reconstructed: in the pregnancy recognition phase about 7% ChA and 10-15% DA conceptuses remain in the population of surviving fetuses. Prenatal selection decreases gradually the frequency of anomalies to the values that are usually found in new-borns. The disappearance of particular anomalies is nonrandom, changing the malformation spectrum continually. The risk of prenatal elimination of the anomaly is not invariably proportional to the degree of phenotype damage of the fetus. Study of the hidden part of prenatal population offers the possibility to cover a considerably larger fraction of the whole reproduction output and supplies new valuable knowledge.

Institute of Preventive and Clinical Medicine Bratislava