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.
This means that you do not menstruate. 1  If you are exclusively breastfeeding, the prolactin levels go high, preventing ovulation for at least six months, according to the book Maternal Child Nursing Care.  If not breastfeeding, you will get your first period after six to eight weeks after delivery.