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.
So are heavy periods that go on 2 weeks or more, and 2-3 months or so without a period. Ovulation gets all messed-up so intervals between periods go wacky. But it's always best to have an obs/gyn referral from your doctor, and get checked out thoroughly so you can rule out any other problem first.