Miscarriage causes range from random chromosomal errors to chronic health conditions, lifestyle factors, and uterine abnormalities — and most are beyond your control. Around 10 to 20 percent of known pregnancies end in early loss, and roughly half of first-trimester miscarriage causes trace back to a chromosomal problem in the embryo. Understanding the real miscarriage causes helps you separate fact from blame, identify what you can change, and know when to seek medical support.
This guide breaks down the most common miscarriage causes, the data behind each one, and the steps that may lower your risk. The facts below come from ACOG patient guidance on early pregnancy loss and current US clinical reviews.
What is a miscarriage?
A miscarriage, also called early pregnancy loss or spontaneous abortion, is the loss of a pregnancy before 20 weeks. About 80 percent of miscarriages occur during the first trimester — usually in the first six to eight weeks. Many happen so early that the woman may not yet know she is pregnant.
Ready to start your project?
Join +450,000 members who find their co-parent or sperm donor
Start free✓ Free ✓ No commitment ✓ Verified
The American College of Obstetricians and Gynecologists defines early pregnancy loss as a nonviable intrauterine pregnancy within the first 12 weeks and 6 days of gestation. After 13 weeks, the risk drops sharply.
What are the main miscarriage causes?
The leading miscarriage causes fall into seven categories: chromosomal abnormalities, maternal health conditions, uterine and cervical problems, hormonal imbalances, age, lifestyle factors, and infections. Most early losses involve chance — not something the parent did or failed to do.
| Cause | Approximate share | When it usually occurs |
|---|---|---|
| Chromosomal abnormalities | 50–60% | Weeks 6–10 |
| Maternal health conditions | 10–15% | Any trimester |
| Uterine or cervical issues | 10–15% | Late first or second trimester |
| Hormonal imbalance (PCOS, low progesterone) | 5–10% | First trimester |
| Lifestyle factors | Variable | Any trimester |
| Infections | Less than 5% | Any trimester |
Chromosomal abnormalities
About 50 to 60 percent of first-trimester losses come from chromosomal errors in the embryo. These happen by chance during fertilisation, when the egg or sperm carries an abnormal number of chromosomes. The body recognises that the embryo cannot develop normally and ends the pregnancy. These genetic events are random and cannot be prevented.
Health conditions that increase risk
Several chronic illnesses are well-documented miscarriage causes. They include uncontrolled diabetes, lupus, antiphospholipid syndrome, untreated thyroid disease (especially hypothyroidism), and uterine infections. These conditions can disrupt blood flow to the uterus or trigger an immune response that interferes with the pregnancy. The good news: when treated, the risk often drops to baseline.
Uterine and cervical abnormalities
Some women have a uterus with an irregular shape — a septate, bicornuate, or unicornuate uterus — that makes implantation harder or limits fetal growth. Fibroids and intrauterine adhesions can also raise the risk. Many women only discover these abnormalities when trying to get pregnant quickly or after a loss.
An incompetent cervix — when the cervix dilates too early, usually between 14 and 23 weeks — is a leading cause of late miscarriage. Treatment options include cervical cerclage and progesterone supplementation.
Polycystic ovary syndrome and hormone imbalances
Polycystic ovary syndrome (PCOS) affects roughly 1 in 10 women of reproductive age. It causes hormonal disruption — high androgen levels, insulin resistance, and irregular ovulation — and modestly increases miscarriage risk. Low progesterone, which is essential to maintain the uterine lining, is another hormonal cause sometimes treated with supplementation.
Maternal age
Age is one of the strongest miscarriage causes. The risk rises sharply after 35:
- Under 30: roughly 10 percent of pregnancies
- Age 35–39: about 20 to 25 percent
- Age 40–44: 40 to 50 percent
- Age 45 and older: 75 to 80 percent
This pattern reflects the increasing rate of chromosomal errors in older eggs. Paternal age over 45 also adds modest risk.
Lifestyle factors among miscarriage causes
Several modifiable behaviours increase miscarriage risk. Drinking 200 mg or more of caffeine daily — about two cups of coffee — roughly doubles the risk compared to women who drink none. Smoking doubles the chance of early loss because nicotine crosses the placenta and reduces oxygen delivery. Alcohol and recreational drug use also raise risk in a dose-dependent way. Both obesity and being underweight affect ovulation, hormone balance, and pregnancy outcomes.
Stress alone has not been shown to cause miscarriage, although chronic stress may influence other risk factors. Learning to manage emotional stress when trying to conceive supports overall fertility.
Listeriosis and other infections
Listeria monocytogenes — found in unpasteurised milk, soft cheeses, deli meats, and unwashed produce — can cause listeriosis. The mother may have only mild symptoms, but the bacterium crosses the placenta and may cause miscarriage, stillbirth, or premature birth. Other infections linked to pregnancy loss include cytomegalovirus, parvovirus B19, Zika, syphilis, and bacterial infections such as mycoplasma hominis and ureaplasma urealyticum.
How to lower your risk
Most miscarriage causes are not preventable, but you can reduce modifiable risks before and during pregnancy:
- Stop smoking, drinking alcohol, and using recreational drugs before conception.
- Limit caffeine to under 200 mg per day.
- Maintain a healthy weight (BMI 18.5–24.9).
- Treat chronic conditions such as diabetes and thyroid disease before pregnancy.
- Take 400–800 mcg of folic acid daily for at least one month before conception.
- Avoid high-risk foods (raw fish, soft cheeses, deli meats, unpasteurised dairy).
- Get vaccinated against rubella and chickenpox if not already immune.
If you have had two or more consecutive losses, ask a fertility specialist for a recurrent miscarriage workup. Connecting with a community of future parents tracking their ovulation cycle can also help during this journey.
Frequently asked questions about miscarriage causes
Can stress cause a miscarriage?
Everyday stress does not cause miscarriage. Studies have not found a direct link between normal life stress and pregnancy loss. However, severe chronic stress may indirectly affect hormones and behaviours that influence pregnancy.
What are the most common miscarriage causes in the first trimester?
Chromosomal abnormalities account for most first-trimester miscarriage causes — about 50 to 60 percent. Maternal age, hormonal imbalances, and underlying health conditions follow.
Can exercise or sex cause a miscarriage?
No. Moderate exercise and sexual activity are safe in a normal pregnancy. ACOG specifically notes that everyday activities — including falls, frights, or stress — are very rarely the reason a pregnancy ends.
How soon can I try again after a miscarriage?
Research published by the US National Library of Medicine on early pregnancy loss shows that conceiving within three months of a loss is safe and may even improve live-birth rates. Wait until bleeding has stopped and you feel ready emotionally.
When should I see a doctor?
See a fertility specialist after two consecutive miscarriages — earlier if you are over 35. A workup typically includes genetic testing, hormonal panels, antiphospholipid antibody screening, and a uterine imaging study.
Recovering from pregnancy loss can feel isolating. Join a verified community of future parents and connect with people who truly understand. Sign up free on CoParents.com to find a co-parent, a sperm donor, or simply real support from others on the same journey.