According to the Centers for Disease Control and Prevention, about 32 out of every 1,000 babies born in the US these days are twins. Back in 1980, only 19 out of 1,000 newborns had twin status. Which is not surprising: newborn twins can be a problem. But once you get over the hump of two tiny kids at once, raising them can actually be easier than having kids of different ages. Read on to learn more about twins.

Do you think there are only fraternal and identical twins? While they are by far the most common, there are actually other types of twins. For example, you might have mirror twins—identical twins from an egg that split in two later than usual—who have “mirror images” of birthmarks and signs.

In very, very rare cases, your twins can be conceived through superfetation, when eggs from two different menstrual cycles are released, fertilized, and then implanted in the uterus. These twins can be born on different days, and sometimes even weeks or months apart. According to a study by University College London, fifteen percent of parents were mistakenly told that their identical twins were fraternal. Why the confusion? Most identical twins share one amniotic sac and one placenta, but 25-30% actually have two separate placentas and amniotic sacs.

However, not all doctors are aware of this fact, according to a survey of members of the American College of Obstetricians and Gynecologists. If you have same-sex fraternal twins that are very similar to each other, there is a chance they might be identical. Talk to your doctor about a DNA test if you’re curious enough to know for sure.