What Causes Miscarriage?

Woman sad about her miscarriage

When you get the diagnosis that you’re having a miscarriage, it’s common for one of your first thoughts to be “why did this happen?” Because pregnancy loss isn’t talked about enough, most people don’t know what causes miscarriage to happen. Depending on when in your pregnancy the loss happens, doctors can narrow the list of possible reasons to just a handful.

first trimester

Miscarriages are most common in the first trimester (the first 12 weeks of pregnancy). This is when about 80% of miscarriages happen. By far, the most common reason for an early miscarriage is a genetic or chromosomal problem with the pregnancy. It could be one abnormal egg, or one abnormal sperm, when the rest of your eggs and sperm are healthy. Or the egg and sperm were perfectly healthy. But when they got together and the sperm fertilized the egg, there was some sort of error. As the cells were dividing, a chromosome may have been duplicated, or a piece of a chromosome broke off. When this happens, your body often recognizes that a healthy baby won’t grow, so the pregnancy stops developing altogether.

It’s like the difference between cooking and baking. You can toss together a lot of different kinds of ingredients to cook an omelet or a casserole, and the dish will taste delicious. But baking is really precise. If you get the proportions of flour and sugar and salt and baking powder off even a little, the dish may be a disaster. Growing a baby is much more like baking than cooking. The ingredients to make a human are incredibly complex, and it doesn’t take much of a genetic error to doom the whole pregnancy. And these mistakes in the making happen well before you know you’re pregnant. There’s nothing you can do to stop them from happening. (So don’t feel guilty.)

The other categories of reasons why pregnancies are lost in the first few months are:

  • Problems with the inside of the uterus or cervix
  • Serious and uncontrolled medical conditions in the pregnant person
  • Infections
second trimester

The reasons for miscarriage in the middle three months of pregnancy are similar to the earlier reasons:

  • Abnormalities in the baby that are:
    • Chromosomal (too many or too few copies of a chromosome),
    • Structural (like lack of development of the brain), or
    • Syndromic (a group of problems commonly found together)
  • Medical illness in the pregnant person
  • Problems with the uterus or cervix
  • Premature labor, which may be caused by an infection or breaking your water
third trimester

Stillbirth, or the death of a fetus after 20 weeks, has a lot of known causes. While doctors can’t find the reason for the loss for about half of pregnancies, these are the causes they can find:

  • Placental and umbilical cord problems
  • Medical illness and pregnancy complications, including preeclampsia, diabetes, lupus, and being pregnant longer than 41 weeks
  • Infections such as cytomegalovirus (CMV), listeriosis, or toxoplasmosis
  • Fetal conditions, that include birth defects and growth restriction

There’s a much longer list of things that DON’T cause a miscarriage. And you can find more information about miscarriage and pregnancy loss of all kinds in my book. Please know that the vast majority of the time, there’s nothing that you could have done to prevent your loss.