A family friend of mine is a quadruplet (born in the early 60's), and the doctors thought the four of them were just twins. My uncle and aunt are twins (born in the 60's), but my grandparents didn't know they were expecting twins until they were born.
As a woman who has never been pregnant, I just can't imagine not knowing I'm expecting multiples.
Prior to ultrasound, physicians used the stethoscope, with varying levels of skill. Additionally, and horrifically, one of the earlier uses of x-rays was to confirm pregnancy.
Brecher and Brecher "The Rays" relates: "An X-ray picture of a fetus in utero had been secured as early as February of 1896--two months after Roentgen's discovery--and it had become common practice to use X rays to detect multiple births or abnormal conditions in the uterus, and to clarify the outlines of the mother's pelvis to aid in delivery."