In fact, we also develop the gonads. So by the end of the first trimester we basically know exactly what the sex of that fetus is because the gonads are essentially formed. So if it's a female, we'll have ovaries. If it's a male, we'll have testes. Now let's move on to the second trimester. By the way, the first trimester begins with that last period, with that last menstrual cycle and ends at the 13th week following fertilization, essentially.