"Low estrogen and ovulation stops due to high prolactin," Brodman explains. "This essentially prevents ovulation, which is practical: new moms need to care for their babies and allow space between pregnancies. There's no sure way to know exactly when you'll start to ovulate again, so don't rely on nursing as contraception." That means you can get pregnant on your period, so use protection. Once ovulation occurs, your period will come two weeks later (unless you didn't use contraception and conceived), she says.