13 Possible Miscarriage Causes

THE MENSTRUAL CYCLELIFESTYLEFERTILITYMENTAL HEALTH
Written By Lisa Burnley

Pregnancy loss at any stage is a devastating experience — and one that often leaves us asking, “What went wrong?”

While we may not always get a definitive answer, there are a number of known risk factors that could contribute to miscarriage. It’s important to remember that these are not causes of blame, but rather points of awareness that may guide further testing or support.

Let’s explore 13 possible causes, and in our next post, we’ll share strategies for reducing these risks and supporting your body through early pregnancy.

Note: If you’ve experienced a miscarriage, please be gentle with yourself. These risk factors are not absolutes — and in many cases, causes remain unknown.

1. Chromosomal Abnormalities

Most miscarriages are caused by aneuploidy (an extra or missing chromosome). Common forms include:

  • Trisomy 16 (21.8%)

  • Trisomy 22 (17.9%)

  • Trisomy 21 (10%)

These errors become more common with age, which is why we recommend preconception care for women over 35.

2. Cigarette Smoking

  • A 1% increased risk per cigarette smoked during pregnancy

  • A partner’s smoking increases miscarriage risk by 17%

  • Smoking before pregnancy is also linked to birth defects

3. Alcohol Consumption

  • Each week of drinking during weeks 5–10 increases miscarriage risk by 8%

  • Alcohol in the 3 months prior to conception (for both partners) increases congenital heart defect risk

  • Risk is higher for women over 30

4. Caffeine Consumption

  • Miscarriage risk rises 19% per 150mg/day of caffeine

  • Even 2 cups per day increases risk by 8%

  • Caffeine crosses the placenta and reduces blood flow

5. Obesity

  • Obese women have a 25–37% higher miscarriage risk

  • Risk of stillbirth also increases with BMI

6. Poorly Controlled Diabetes or Insulin Resistance

  • Seen commonly in PCOS

  • Insulin is toxic to placental cells during early pregnancy

  • This sensitivity can cause miscarriage if not well-managed

7. Thyroid Dysfunction

  • Optimal TSH in pregnancy: 0.2–2.5 mU/L

  • Risk increases 1.8x at 4.5–10 mU/L, and 3.95x above 10 mU/L

8. Anatomical or Cervical Issues

  • Weak cervix or uterine abnormalities (e.g. septate uterus, fibroids) can lead to miscarriage

  • Seen in 15% of women with 3 or more losses

9. Antiphospholipid Syndrome (APS)

  • Causes clots that prevent the embryo from adhering

  • Usually only tested after 3 or more miscarriages

10. Balanced Chromosomal Translocations

  • Occurs when parts of chromosomes swap places

  • One or both parents may carry this

  • Responsible for 20–50% of miscarriages in affected couples

  • 4.5% of all recurrent miscarriages stem from this cause

11. Bacterial Vaginosis (BV)

  • BV increases miscarriage risk by 2–3x

  • Most associated with spontaneous miscarriage in the past 6 months

12. Thrombophilias (e.g. Factor V Leiden)

  • Causes excessive blood clotting

  • Can impair implantation

  • May play a role in 40–50% of recurrent miscarriage cases

13. Natural Killer (NK) Cells & Immune Dysfunction

  • Uterine NK cells are involved in implantation

  • Some studies show higher NK levels in women with recurrent miscarriage

  • Blood tests may not be helpful — uterine biopsy is required for accurate testing

📚 A 2022 meta-analysis found a strong association between elevated uterine NK cells and recurrent pregnancy loss.

We’re Here to Support You

If you’re navigating miscarriage, please know you are not alone. We would be honoured to support you with personalised care, testing options, and emotional support.

Our follow-up post will explore practical ways to reduce miscarriage risk through lifestyle, testing, and natural medicine.

Lisa Burnley
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