While pregnant with twins I learned a lot about twins, how they happen, why they happen and that they're much more common than I ever thought.
How they happen - either mom drops two eggs that each get fertilized (fraternal twins) or one fertilized egg splits for some unknown reason (identical twins).
Why they happen - (This applies to fraternal twins only) Heredity, if someone in the woman's family has had twins she is more likely to have twins. Age, the older a woman gets the more likely she is to drop more than one egg at a time. Fertility treatments.
Twins happen every 1 in 30 births now. Much higher than 30 years ago. In the last 3 decades the occurrence of twins has risen 72%.
On the same day I delivered our twins, Ministry Saint Michael's delivered another set earlier in the day and sent a mother expecting twins to Marshfield because they were too early.
Since having twins I've met so many people who are twins or have twins. I've even met a few people who have 2 sets of twins. They say that once you have twins you're more likely to have them again.
Though twins are pretty common these days there are some cases of twins that are still quite rare like the ones in this video: