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.
Even though this is less likely if you are exclusively breastfeeding (day and night, no solids, no bottles, at least 8 times a day, never going more than 4 hours during the day or 6 hours at night without feeding), have not had a period, and your baby is younger than 6 months old, it is still possible.