Twins and multiples have been around forever, but people are often still mystified as to how they happen. And with assisted reproduction, the incidence of twins and triplets or higher multiples has increased. How do multiples happen?
I'm going to talk about twins, but I mean multiples of any number. Fifty years ago or so, a mother of twins often found out she was having two babies at birth. Can you imagine that? You think you're done when one baby is delivered and then another one comes along, too! Sometimes the two heartbeats were heard during pregnancy, but not always. Today with ultrasound technology, a mother knows she's having multiples in advance.
People talk about twins "running in the family." That is true only of fraternal twins, which arise from different eggs. The likelihood of releasing more than one egg with ovulation can be genetic. Identical twins, however, are not due to genetics--they truly just happen when one eggs divides an extra time. Identical twins also typically look alike and are the same gender whereas fraternal twins may or may not resemble each other, and they may be different genders.
As women age, the likelihood of having fraternal twins increases. More than one egg is released with ovulation, which may be Mother Nature's way of making sure conception occurs.
Then there are some rare types of twins, such as superfecundation, where the mother ovulates more than once in the same cycle. This is only clear when she also has more than one partner, so the babies have different fathers, but are born at the same time!
The most common type of multiple is twins, which is good because higher order multiples become more risky for pre-term labor. Unless you're using assisted fertility, there's no way to know if you're ovulating more than one egg in a cycle. Ovulation predictor kits can only tell you that you're having an LH surge, which usually leads to ovulation. Finding out you're having twins at the typical 12-week ultrasound would be a nice surprise.