Most women find out if they are having twins during their first trimester of pregnancy, according to Baby Center. As early as nine to 14 weeks, an ultrasound technician may be able to determine if a woman is having twins and if the twins are monozygotic or dizygotic.
Monozygotic twins share the same placenta, while dizygotic twins have separate placentas, states Baby Center. By the time a woman reaches her 18th to 20th week of pregnancy, the gender of the babies may be determined. If the genders are the same and the babies are monozygotic, then they are identical twins. If the genders are the same and the babies are dizygotic, they may be either identical or fraternal twins.