Why Miscarriages Happen: The Real Causes Most Doctors Don't Fully Explain

After a miscarriage, the question every woman asks is: why? And in too many cases, the answer she gets is "chromosomal abnormality — there was nothing you could have done." That's often true. But it's not always the complete story. Here's what actually causes miscarriages, and what that means for preventing future ones.

"Chromosomal abnormality" is the most common cause of miscarriage — but it's not always the full explanation, and it's not always out of your control.

Chromosomal Abnormalities: The Most Common Cause

Approximately 50–60% of first-trimester miscarriages are caused by chromosomal abnormalities in the embryo — the wrong number of chromosomes at fertilization, which usually results in a pregnancy that can't develop past a certain point. This rate increases with maternal age and reflects the chromosomal error rate in eggs and, to a lesser extent, sperm.

When a miscarriage is caused by a chromosomal abnormality, it typically could not have been sustained — the body is doing exactly what it should do. This is genuinely not something you caused, and it's not something that will necessarily recur. However — and this is important — improving egg quality through targeted supplementation and physiological optimization reduces the rate of chromosomal errors. The follicular environment affects how accurately chromosomal segregation occurs during meiosis. This is not a minor point: optimizing egg quality is one real lever on chromosomal miscarriage rates.

Hormonal Causes: More Common Than Diagnosed

Progesterone insufficiency is a frequently missed contributor to early pregnancy loss. The corpus luteum produces progesterone after ovulation, which maintains the uterine lining and supports early implantation until the placenta takes over at 8–10 weeks. When progesterone levels are inadequate — either because the corpus luteum is weak or because the luteal phase is short — early pregnancy becomes fragile.

The diagnostic criteria for "low progesterone" in the medical system often miss borderline cases. A progesterone level of 10 ng/mL at 7 DPO is technically within the range of "ovulatory" — but it may not be sufficient to sustain a pregnancy. Subclinical progesterone insufficiency is real, clinically significant, and often not caught.

Thyroid antibodies — present in Hashimoto's thyroiditis — are independently associated with a 2–3x increased risk of miscarriage even when TSH is normal. This is a critical screening gap: many women have thyroid antibodies and normal TSH, never get antibody testing, and are told their thyroid is fine.

Anatomical Factors

Uterine abnormalities — submucosal fibroids, uterine polyps, a uterine septum, or intrauterine adhesions — can interfere with implantation or early placental development. These are typically identified through hysteroscopy or saline infusion sonography and are often correctable. If you've had more than one miscarriage, a uterine cavity evaluation is essential.

Immune Factors

Antiphospholipid antibody syndrome (APS) is an autoimmune condition that causes blood clots in placental vessels, leading to pregnancy loss — often in the second trimester, though it can cause recurrent first-trimester losses as well. It's treatable with aspirin and heparin. And it is significantly underscreened in women with recurrent pregnancy loss. This is a test that should be routine after two or more miscarriages and often isn't.

Male Factor and DNA Fragmentation

High sperm DNA fragmentation can cause embryos that fertilize and implant but fail to develop normally — resulting in early miscarriage even when a standard semen analysis looks normal. This is not tested in a routine semen analysis; it requires a specific DNA fragmentation test (DFI). For recurrent pregnancy loss where other causes have been ruled out, paternal DFI testing is worthwhile.

"A standard semen analysis can look completely normal while high DNA fragmentation silently drives recurrent miscarriage. It's one of the most underdiagnosed factors in pregnancy loss — and one of the most fixable."

Frequently Asked Questions

What tests should I ask for after a miscarriage?

After a first miscarriage, standard recommendations vary. After two or more, most specialists agree on: chromosomal analysis of the pregnancy tissue (products of conception), uterine cavity evaluation (SIS or hysteroscopy), complete hormonal panel (day 3 FSH/LH/estradiol, AMH, TSH with TPO antibodies, prolactin, luteal phase progesterone), antiphospholipid antibody panel, and karyotype for both partners. Some also test for thrombophilia depending on clinical picture.

Is it true that once you've had a miscarriage, you're more likely to have another?

One miscarriage increases your risk slightly — but the absolute risk of recurrence is still lower than the risk of a first miscarriage. After two miscarriages, the risk of a third increases more meaningfully, which is why thorough evaluation and intervention after two (or even one in women over 38) is so important. The pattern matters: random chromosome errors are a different picture from progesterone insufficiency or thyroid antibodies.

Can stress cause a miscarriage?

Extreme acute physiological stress — severe illness, major physical trauma — can. Psychological stress, while clearly affecting fertility through HPA axis dysregulation, has not been shown to directly cause miscarriage in healthy pregnancies. That said, chronic stress affects the hormonal environment that supports early pregnancy — progesterone production, implantation signaling. The connection is real but indirect, not "you stressed your way into a miscarriage."

What can I do to reduce miscarriage risk in future pregnancies?

Address identified causes specifically. For chromosomal miscarriages, egg quality optimization through CoQ10, antioxidants, and metabolic health improvement reduces chromosomal error rates. For progesterone insufficiency, supplemental progesterone in early pregnancy is often used and is low-risk. For Hashimoto's antibodies, thyroid optimization and sometimes low-dose aspirin. For APS, aspirin and heparin. For uterine abnormalities, surgical correction where appropriate. The key is knowing what's causing the losses and addressing that specifically.

How long should I wait before trying again after a miscarriage?

Physically, the uterus is typically ready for the next conception within one to two cycles — there's no evidence that waiting longer improves outcomes. Emotionally, the timing is yours to decide. The old recommendation to "wait three months" has largely been replaced with "whenever you feel ready physically and emotionally." If the miscarriage required D&C, your RE will give you specific guidance on timing.

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Written by Kirsten Karchmer, reproductive medicine practitioner with 25 years of clinical experience and 10,000+ credited pregnancies, and author of The Road to Better Fertility.