Why do miscarriages happen?

We often hear that miscarriages are common, and that’s true, unfortunately. That doesn’t make them any less stressful though. A miscarriage is often tough and understanding what has happened, as well as when it’s time to seek help, is important to feel a little more in control.

Different types of miscarriages

Although they all mean the same thing - that unfortunately the pregnancy doesn’t lead to a baby - there are different types of miscarriages and therefore also different reasons. Having one miscarriage is very different from having several - not only emotionally but medically too. So, let’s go through the different types of miscarriages...

Early miscarriage

An early miscarriage happens in the first trimester, during the first 12 weeks of pregnancy. It’s estimated that 10-20% of all pregnancies end in a miscarriage and this is by far the most common type. After 12 weeks, the risk for a miscarriage drops significantly.

An early miscarriage can either be spontaneous, meaning you start bleeding, or a so-called missed abortion (MA) where the bleeding doesn’t start by itself (at least not for a few weeks) after the fetus has died. Instead, the miscarriage is discovered via an ultrasound.

Is bleeding always a sign of miscarriage? No, bleeding is not uncommon during pregnancy for a range of reasons - most women who experience vaginal spotting or bleeding in the first trimester go on to have successful pregnancies. However, if you experience continuous bleeding and/or cramping in your abdomen or lower back you should visit a doctor.

Late miscarriage

A late miscarriage happens after the first trimester. It’s much less common than early miscarriages, estimated to happen in 1-2% of pregnancies. When you have a late miscarriage, it entails more than bleeding - since the fetus is much bigger (depending, of course, on which week it stopped growing) you will need to go through labor and giving birth, which can be very distressing. Sometimes labor starts spontaneously, or it needs to be induced when the miscarriage has been discovered via ultrasound.

Recurrent miscarriages

If you’ve experienced more than 3 consecutive miscarriages, you’re one of the unlucky ones diagnosed with “recurrent miscarriages”. The exact definition varies from country to country. In some countries two consecutive miscarriages are considered recurrent, in some they don’t have to be consecutive.

In any definition, it’s an uncommon diagnosis, affecting around 1% of women, but when you’re a part of it, statistics don’t matter much. Please keep close to your heart that a clear majority of those suffering from recurrent miscarriages go on to successfully carry a pregnancy to term. However, the road there can be difficult because we don’t know that much about the cause of this condition and many are left with no explanation.

Chemical pregnancy

A chemical pregnancy is an early pregnancy loss that occurs shortly after implantation. It happens before an ultrasound can detect a fetus, but not too early for a pregnancy test to detect levels of hCG.

Often, the woman doesn’t even know that she’s pregnant this early on and the bleeding is thought to be an ordinary menstruation. However, if you go through IVF or if you're just trying to actively conceive you probably test early; a chemical pregnancy is then when you start to bleed just a few days or a week after the positive test.

Why do miscarriages happen?

Since early, late and recurrent miscarriages are such different things, the potential causes also have to be looked at separately.

Potential reasons for early miscarriage

The most common reason for early miscarriages is that the fetus has abnormal chromosomes, it’s basically not healthy. Since early miscarriages are the most common type of miscarriages, it is the most common reason for miscarriages in general.

“Abnormal chromosomes” may sound serious, but basically not all eggs or sperms have normal chromosomes. This is one of the reasons you don’t become pregnant every time you have sex when you ovulate even if there’s no issue out of the ordinary; unhealthy eggs and sperms most often don’t fertilize or implant at all. If they do, a miscarriage will happen. (Of course, there’s also chromosomal abnormalities that are not serious enough to always cause a miscarriage too, e.g., Trisomi21 also called Down’s Syndrome.)

When doctors say a miscarriage is “bad luck” (not very comforting, we know…) this is what they mean… you were unlucky enough to become pregnant with one of those eggs and/or sperm that didn’t have normal chromosomes. In other words, there is no indication that there’s any issue which would cause you to miscarry again.

There are, however, some factors that increase your risk of having a miscarriage due to chromosomal issues:

  • Maternal age. With age, the number of eggs with normal chromosomes becomes smaller which makes it both more difficult to become pregnant and increases risk for miscarriage.
  • Smoking. Eggs and sperms are cells, and their quality is harmed by everything that causes so called oxidative stress, such as seen in smoking. Smoking, in other words, causes more abnormal eggs and sperms.

There can of course be other reasons for an early miscarriage than abnormal chromosomes; thyroid imbalances, hormonal issues, and blood clotting problems to mention some. But since the absolute majority are caused by chromosomal issues, other reasons are normally not investigated until you have suffered several miscarriages in a row (the risk of chromosomal issues being at play decreases the more miscarriages you go through) unless you already know you have an underlying health condition.

Potential reasons for late miscarriage

Late miscarriages can be due to chromosomal and genetic factors just like early miscarriages. But compared to early miscarriages, late miscarriages more often have another reason. Potential reasons are:

  • Infections
  • Anatomical issues
  • Placental problems
  • Long term health issues
  • Invasive prenatal test

You can read more about each of these potential causes as well as recurrent miscarriages in Tilly's app. There's also a checklist to bring to your doctor's appointment to make sure you get a correct investigation in case of recurrent miscarriages.


Evangelia Elenis, MD, PhD.

This text is fact checked by Evangelia Elenis, MD, PhD. Dr. Elenis is a chief physician in Obstetrics and Gynecology, and a subspecialist in Reproductive Medicine. She is a PhD and affiliated researcher at Uppsala University with postdoctoral studies at Harvard Medical School.

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