Remember, ladies didn't have the early pregnancy test decades ago, so they wouldn't have realized they were pregnant until after the missed period—this is why the miscarriage rate is theoretically greater these days. However, there are many factors other than just time since last period that can affect how far along you are in your pregnancy, including age, weight, previous pregnancies, and medical conditions. So even though you may think it's safe to say that the chance of miscarrying is higher if you haven't had a period yet, this isn't always the case. Here's what scientists know about how long it takes for all embryos to lose their viability: if an embryo makes it to day 12, then it has a good chance of developing into a baby. From day 13 through day 18, almost all embryos will degenerate or die. This includes twins, triplets, and more. Based on this information, most experts agree that a woman should wait at least 12 days after her last menstrual period before having any type of prenatal screening test, such as an ultrasound or blood test. However, studies have shown that some women may not experience much of a change in their body temperature during early pregnancy, which can cause problems determining exactly when they last had their period.