Part one: What are the common causes of miscarriage?

Doctor consoling patient with miscarriage.
Miscarriage affects up to one million women in the United States each year.

Many people do not realize miscarriage is the most common pregnancy complication in the United States. Unfortunately, because we don’t discuss it much, many women have the false idea that a miscarriage is their fault.

As I’ve addressed in previous blogs, this is simply not true. There are many medical reasons why a miscarriage might occur.

This week, I am going to walk through the most common causes of miscarriage. And next week we’ll discuss recent research into antibiotics and miscarriage risk.

Detecting pregnancy & pregnancy loss

Eighty percent of spontaneous miscarriages occur in the first 12 weeks of pregnancy, so I want to focus on causes in the first trimester. And a little information on terminology. The medical term for miscarriage is “spontaneous abortion.” I always make sure women understand that, since the word “abortion” can be misunderstood and women may see it on their own medical records or explanations of insurance benefits.

The first step to diagnosing miscarriage is diagnosing the presence of a pregnancy. As pregnancy tests are able to detect pregnancy at an earlier and earlier stage, we also recognize more and more cases of early miscarriage that previously would have been missed.

We diagnose pregnancy by measuring the level of beta human chorionic gonadotropin ( β HCG) levels. Today’s tests, which you can buy in most drug stores or big-box stores, detect β HCG at very low levels, allowing us to diagnose pregnancy before you miss a period or develop other symptoms.

Because of this development, we can now distinguish between a “clinical pregnancy,” when you experience signs and symptoms that a pregnancy has started, and a “chemical pregnancy,” when the β HCG test is positive but is the only indicator of pregnancy.

I cannot emphasize enough how common miscarriage is, especially in early pregnancy. One recent study followed over 200 women trying to get pregnant and found that about 30 percent of all pregnancies were lost. Two-thirds of those pregnancies never had any clinical signs of pregnancy. The only way women in this group knew they were pregnant was because an early pregnancy test was positive. Before pregnancy tests were this accurate, women would typically think their period just came a little late.

Chromosomal abnormalities

This ultrasound image shows an embryo at six weeks.
This ultrasound image shows an embryo at six weeks.

The most common identifiable (and unavoidable) cause for pregnancy loss is a chromosomal abnormality in the developing fetus or placenta.

In about half of all miscarriages, there is an abnormal amount of chromosomal material. It might be an extra chromosome, as in the case of trisomy 21 or Down syndrome. Sometimes extra material is added to or taken away from a single chromosome. There may even be whole extra sets of chromosomes – in which case, instead of 46 chromosomes, the developing fetus has 69 or even 92 chromosomes in each of its cells.

When these chromosomal abnormalities occur, often times the developing embryo or fetus can’t survive. When your body recognizes this, the pregnancy is lost. 

Older women tend to have a higher risk of miscarriage, which correlates with a higher risk of chromosomal abnormalities in their eggs. This is also why older women tend to be at higher risk for having babies with Down syndrome.

Abnormal uterus

When a fertized egg meets an abnormal uterine environment, there is an increased risk for spontaneous pregnancy loss in the first trimester.

For example, if scar tissue has formed after a D&C or a prior endometrial ablation it may be difficult for the developing pregnancy to find a good place to implant within the uterus.

Uterine fibroids can distort the uterine cavity and contribute to early pregnancy loss, especially if the early pregnancy has implanted directly over the fibroid, which makes it difficult for it to get good blood supply to the growing tissue.

Congenital conditions, in which the uterus is formed abnormally from birth, can also cause problems. In women with a bicornuate uterus, where the uterus is divided into two halves, up to 70 percent of pregnancies end in miscarriage.

Maternal medical problems and treatments

There are other conditions that can contribute to spontaneous pregnancy loss as well. If the mother develops a serious infection and has sepsis or has other medical problems such as uncontrolled diabetes, the risk for miscarriage increases.

Still other disorders, such as antiphospholipid antibody syndrome (which can accompany medical problems such as systemic lupus erythematous and other connective tissue disorders) can also increase miscarriage risk. In this situation, small blood clots in the placenta may contribute to pregnancy loss.

Treatment with certain chemotherapies or radiation for cancer in early pregnancy can also increase the risk of miscarriage.

If you have had a miscarriage or have a medical problem that might make you at higher risk, I encourage you to talk to your physician. There may be treatments or interventions that increase the likelihood of a successful pregnancy.

Looking ahead: Are antibiotics linked to miscarriages?

There is some evidence that, in addition to these common causes of miscarriage, taking certain antibiotics can increase the risk of miscarriage in the first trimester. Next week, I’ll explain a recent study on this topic and the implications for pregnant women.

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