Women do not become pregnant immediately. Out of 100 couples wishing to have a baby, only 25 % conceive within the first month. The remaining 75 % must try again the next month. During this second month, a further quarter of those who did not succeed the first time will conceive, while the others must keep on trying... On average, couples take seven months to achieve a pregnancy.

Practically all female mammals become pregnant very quickly once the breeding season begins, but humans are different in this respect.

The low success rate among humans can be explained by early embryo mortality: most couples produce a fertilized egg, but in many cases it dies within the next two weeks, often because of genetic defects. The egg is discharged during menstruation, and as the woman's period occurs at the normal time, she not aware that fertilization has occurred. Embryo mortality at this early stage is very high, well above the rate of spontaneous abortion observed beyond two weeks of pregnancy.