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

The most common cause of first-trimester miscarriage is a chromosomal abnormality 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. Researchers are studying whether supplements like CoQ10 may support egg health, but no study has shown they lower the rate of chromosomal errors.

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. (Thangaratinam, BMJ 2011) 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 is linked to higher miscarriage rates, even when a standard semen analysis looks normal (Robinson, Hum Reprod 2012). 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 DNA fragmentation is high, and high fragmentation is linked to recurrent miscarriage."

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, there's no proven way to lower the error rate yet, though researchers are studying whether CoQ10, antioxidants, and metabolic health may support egg health. 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. (Kangatharan, 2017) (Schliep, Obstet Gynecol 2016) 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.

How does the Conceivable system actually work?

Conceivable combines three things: personalized supplement packs built from your quiz results and health data, an AI care team of 7 specialists (led by Kai, your fertility coordinator) who help you understand your pattern and can suggest a pack, and the Halo Ring for continuous biometric tracking. The system is built on published reproductive-health research and 20 years of clinical practice. It starts at $14.99/month.

How do I know which supplements I actually need?

Take the free 2-minute Conceivable quiz. It analyzes your cycle patterns, energy, stress, digestion, and health history to identify the specific nutrients your body needs, not a generic prenatal, but a protocol built for exactly where you are right now.

Do I need the Halo Ring to use Conceivable?

No. The Halo Ring is optional and adds overnight tracking of temperature, HRV and sleep, which Kai uses to help you understand your pattern and suggest a pack. But the personalized supplement packs and AI care team work without it. The ring is a one-time $250 purchase with no subscription required.

Written by Kirsten Karchmer, founder of Conceivable. She has worked in reproductive health for 26 years, including 20 years in clinical practice, and is a former board-certified reproductive acupuncturist (ABORM) and the author of Seeing Red (Simon & Schuster).

Sources

• Thangaratinam S et al. Association between thyroid autoantibodies and miscarriage and preterm birth: meta-analysis of evidence. BMJ. 2011.
• Kangatharan C, Labram S, Bhattacharya S. Interpregnancy interval following miscarriage and adverse pregnancy outcomes: systematic review and meta-analysis. Hum Reprod Update. 2017.
• Schliep KC et al. Trying to Conceive After an Early Pregnancy Loss: An Assessment on How Long Couples Should Wait. Obstet Gynecol. 2016.