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.
First off, if you dont want to get pregnant then no matter what dont have unprotected sex. A condom is alot cheaper than a baby! Also, normally you are not fertile right when you get off your period. If you have a normal cycle, you should ovulate 14 days after the first day of your last period. Every woman is different though and ovulate at different times, so there is always a chance. Remember though sperm can live inside you and wait till you ovulate anywhere from 3-7 days. Use protection!