Ultrasound today can show an embryo five weeks after the last menstrual period. We could see two. Two small, black and white and gray living entities with no discernible activity, except for the heartbeats. Even then we knew that the twins were non-identical. The incidence of multiple pregnancies in people who go in for assisted reproduction, is high.
Twins are very common and often there are triplets and quadruplets.
In fact our ultrasonologist heaved a sigh of relief when he saw that it was a twin pregnancy. Thank God, there aren't more. Often, when there are more than two pregnancies, the ultrasonologist is called in to play God. The euphemistic term used is embryo reduction, which is a procedure where, for the safety of the mother and the other embryos, the doctor injects a drug called methotrexate into the hearts of the extra embryos and terminates them. Technically, with practice, it is not a very demanding procedure, but emotionally its tough, both for the parents and the doctor.