The genotype of the mother and the fetus influences the efficacy of a teratogen. For example, genotype determines the prevalence of cleft palate in inbred strains of mice whose mothers are administered cortisol during pregnancy.
Most women will not start ovulating in the first six weeks after giving birth.1 Breastfeeding typically delays the onset of ovulation.2 Depending on the intensity of breastfeeding, it can be several months or over a year before you regain your fertility.
If you are breastfeeding, you may ovulate as early as nine weeks or 10 weeks after giving birth. Your period would then start two weeks later, as early as 11 weeks or 12 weeks after giving birth. Or it may be over a year before you start ovulating again, and your periods return.