In the past, before ultrasound examinations were a common part of prenatal care, parents were sometimes surprised when two babies were born even though only one fetal heartbeat was heard during the pregnancy.
Today, a routine prenatal ultrasound examination can identify a twin pregnancy. If two fetuses are seen by ultrasound, the woman is expecting twins. Although rare, ultrasound pictures can at times be misleading or misinterpreted, especially if they are taken very early in the pregnancy. It is possible that two fetuses could be positioned in such a way that one is directly in front of the other, blocking the second fetus from view. Such “hidden twins” are more likely to occur when the twins are monochorionic (have one chorion, the outer fetal membrane) and positioned closely in their single chorionic sac. However, after 20 weeks of pregnancy, the fetuses have grown enough that a second fetus would be clearly visible on ultrasound, and it would be very unlikely that there is another baby hidden in the womb.