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Why do miscarriages happen?


For many people, the sudden end of a pregnancy gives rise to a thousand thoughts and feelings. Suddenly, you’re not sure what this means about your body or your ability to have a healthy pregnancy, and you certainly aren’t sure what might happen next. Of the questions that patients ask me, the first is almost always, “Why did this happen to me?”

A person having a miscarriage joins a club that they never intended to sign up for. This club is full of amazing people from all different walks of life who are bonded together by this similar experience. Everyone’s miscarriage journey is different, but you and the others that came before you all experience the physical and emotional loss of a pregnancy ending.

This is a bigger club than you realize. Around 1 in 4 recognized pregnancies ends in a miscarriage — and it may be as many as half the pregnancies, since many people miscarry before they realize they are pregnant. At those rates, about a quarter of women will experience a miscarriage during their lifetimes.

This means you absolutely know at least one person who has had a miscarriage, even if no one has told you they’ve had one. Miscarriages happen to people in every country and every situation in life. Healthy or sick. Young or old. Happy or sad or unsure about the pregnancy. There are risk factors for miscarriages, but it’s important to know that most pregnancy losses happen to completely healthy people.

Here are the most common risk factors for pregnancy loss:

Who you are:

  • Age. Being over 35 or having a male partner over 40.
  • Weight. Being underweight or overweight is associated with miscarriage, but it’s unclear if weight itself directly increases the risk.

What you do:

  • Lifestyle factors. Smoking 10 cigarettes or more a day, drinking alcohol, or using nonprescribed substances, particularly cocaine.
  • Exposure to radiation and toxic substances. Having a job that exposes you to toxins, like pesticides, heavy metals (lead, mercury, arsenic), solvents (toluene and benzene), ionizing radiation and some chemotherapy agents.
  • Extensive lifting at work. Regularly lifting over 220 pounds a day. If your job or workplace poses a risk to you during pregnancy, your doctor can write a letter to your employer. Your employer should make accommodations for you.

Pregnancy complications:

  • Multiple gestation. Twins, triplets, or more.
  • Pregnant with an IUD in place. Intrauterine devices (IUDs) are highly effective forms of birth control, but if the device fails, the pregnancy is at a higher risk of loss.
  • Invasive pregnancy testing. Chorionic villus sampling (CVS) and amniocentesis tests to determine if the baby is healthy can uncommonly cause miscarriage.
  • Infections. Rubella, chickenpox, foodborne infections (listeriosis, salmonellosis).
  • High-velocity trauma, like a serious motor vehicle accident or major fall; or low-velocity trauma, like getting hit in the belly during a low-speed motor vehicle accident in the second or third trimester.

Unclear associations, due to poor or conflicting data:

  • Night shift work. Some studies (but not all) have shown that working a night shift affects long-term health and increases the risk of miscarriage.
  • Taking medication. Certain drugs, including oral medication used to treat a yeast infection, may increase early miscarriage risk. Ask your doctor if there are drugs or supplements you shouldn’t take while pregnant.

The average person who wants to be pregnant and have more than one child may be pregnant many times in a lifetime. The odds are actually higher that you’re going to have at least one miscarriage than that you won’t have any. It’s especially sad when a miscarriage happens in a first pregnancy because of all the doubts it might raise. But you’re no more or less likely to have bad luck on your first pregnancy than your fourth.

There are also plenty of things that do not cause a miscarriage. Here’s a short but by no means definitive list:

Exercise, heavy lifting or physical exertion
Don’t worry about the fact that you vacuumed your house, moved a few heavy boxes, or lifted your niece or nephew. Even if you have a job that involves lifting mattresses or pushing heavy carts. Even if you bench-pressed 150 pounds on a few occasions or ran a marathon (which is amazing). No amount of typical physical activity can cause a miscarriage.

There’s no link between miscarriage and full-time employment, standing more than six hours a day, or an average amount of lifting. While some jobs may be associated with a higher risk, it’s unlikely your job had anything to do with your miscarriage.

Screen time
Working at a computer all day or spending a lot of time on your social media accounts is not associated with miscarriage. The electromagnetic fields from computer screens are weak. While staring at a screen all day may not be good for your eyesight or your relationships, it doesn’t threaten your pregnancy.

Air travel
Cabin pressurization isn’t associated with higher miscarriage risk. However, if you do fly while you’re pregnant, be sure to regularly get up and walk the aisle to prevent blood clots in your legs. Pregnancy does increase your risk of those.

There are myths about a shock or fright causing a miscarriage. That’s not true, either. Enjoy all the horror movies you want.

I don’t care how vigorous the sex was, how athletic or in what position. In fact, I hope all those things were superfun. No amount of sex (or an orgasm, with or without intercourse) can disrupt your pregnancy.

Moderate caffeine consumption (2 cups of coffee or 3 to 5 cans of soda a day) is OK while pregnant.

Tampons stay in your vagina and go nowhere near the baby inside the uterus.

Hormonal birth control
Hormonal contraception doesn’t fundamentally change your ovaries, your eggs or your uterus, and it has no impact on the health of your pregnancies. While the injection can delay your periods from returning for up to a year, you’ll still be able to have a healthy pregnancy once the hormonal effects wear off. If your hormonal birth control failed and you became pregnant while using it, there is no increased risk of miscarriage.

An intrauterine device (IUD)
Similarly, having used an IUD in the past doesn’t increase your risk of miscarriage. The only association with an IUD and miscarriage is in the rare instances when you become pregnant with an IUD in place. These pregnancies have a high risk of miscarriage, especially if the IUD is not removed.

A past abortion
It isn’t uncommon for people to feel that their decisions about past pregnancies are the reason for a present-day miscarriage. Medically, a past abortion — even if you’ve had more than one — has no impact on your pregnancies in the future. You made the best decision for yourself that you could at the time, and an abortion does not damage your uterus so that you can’t get pregnant again. The universe is not sending you a message.

Morning sickness
My patients have asked me if their babies were nutritionally deprived because they were nauseous all the time and couldn’t eat or were vomiting throughout the day. No matter how much you vomited or how little you ate, morning sickness does not lead to miscarriage. In fact, it’s associated with high pregnancy hormone levels that tend to indicate healthier pregnancies.

A flu shot
The effects of flu vaccines on pregnancies have been heavily researched. Despite what you may have heard, getting the flu vaccine during pregnancy is not only safe, but also highly recommended. If you were to get the flu while pregnant, you would have a higher risk of serious illness — and even death — than getting the flu at any other time. Getting the flu vaccine won’t cause a miscarriage and will keep you and your baby safe, both before and after birth.

Bottom line: You did nothing wrong. You did nothing to make this happen.

An excerpt from Your Guide to Miscarriage and Pregnancy Loss

Kate White, M.D., OB-GYN

Dr. White, author of Your Guide to Miscarriage & Pregnancy Loss, is an associate professor of obstetrics and gynecology at the Boston University School of Medicine and the vice chair of academics in the OB-GYN department at Boston Medical Center. She is a fellow of the American College of Obstetricians and Gynecologists, a fellow of the Society of Family Planning, and a member of the American Public Health Association. As a board-certified OB-GYN, mother and pregnancy loss survivor, Dr. Kate has been caring for women for more than 20 years, helping them navigate every stage leading up to menopause, including periods, childbirth and pregnancy loss. She also conducts research in contraception, has been continuously grant funded for 15 years, and frequently lectures regionally and nationally on topics related to women’s reproductive health. Dr. Kate lives outside of Boston with her husband and their three children.

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