Some researchers believe ultrasound should not be done in the first 8 to 10 weeks of pregnancy. Parents who are informed of twins at this point may be distraught to learn later that one of the babies will not be born. Triplets occur once in every 8000 deliveries. Many doctors never deliver or participate in the delivery of triplets in their medical careers. I have been fortunate in my medical practice to deliver two sets of triplets.
Some families are more blessed than others.
In one case I know of, a woman had three single births. We have known for a long time that fertility drugs increase the chance of multiple pregnancies. Several different medications are used to treat infertility. Each one affects your chances of conceiving more than one fetus to a different degree. It increases the chance of multiple fetuses somewhat less than other medications.