Many parents search for miscarriage causes with one question looping in their head: “Did I do something wrong?” A coffee, a workout, a short flight, an argument – everything can suddenly feel suspicious. Yet most early losses come from biology that no one can steer: genetic errors, implantation that does not progress, or health conditions that quietly interfere with the placenta.
Clear information can steady the ground: what “miscarriage” means, how bleeding is assessed, the most frequent miscarriage causes, and which factors are actually modifiable.
What a miscarriage is (including “chemical” pregnancy)
A miscarriage (early pregnancy loss) is the spontaneous end of a pregnancy before viability outside the uterus. Many systems use 20 weeks, some use 24 weeks.
A very early loss may happen before anything is visible on ultrasound. A pregnancy test turns positive because beta-hCG rises briefly, then falls, and bleeding occurs – often close to when a period was expected. This is commonly called a chemical pregnancy.
Terms you may hear:
- Anembryonic pregnancy (blighted ovum): gestational sac seen, no embryo visible.
- Missed miscarriage: development stopped but bleeding may not start right away.
- Threatened miscarriage: bleeding/cramps with a closed cervix, pregnancy may continue.
A key point: for most first-trimester cases, miscarriage causes are not sex, moderate exercise, a single stressful moment, or a “wrong movement.”
Miscarriage vs stillbirth, ectopic pregnancy, and molar pregnancy
- Miscarriage: early loss.
- Stillbirth: fetal death later in pregnancy (often from 24 weeks, definitions vary).
- Ectopic pregnancy: implantation outside the uterus (usually a tube). It cannot continue and can cause internal bleeding.
- Molar pregnancy: abnormal placental tissue growth, hCG is often high and ultrasound looks distinctive.
When miscarriage happens most often
Most miscarriages occur in the first trimester (up to 12-13 weeks). Second-trimester losses are less common, when they occur, clinicians think more about placenta, infection, uterus/cervix, and maternal conditions – different miscarriage causes than in very early loss.
How common miscarriage is (and what risk looks like)
- Around 10-20% of known pregnancies end in miscarriage.
- The true number is higher because many very early losses occur before confirmation.
- About 80% happen by 13 weeks.
Age and risk
Risk rises with maternal age, mainly because chromosome errors in eggs become more frequent:
- 20s: ~12-15%
- 35-39: ~18%
- 40-44: ~37-40%
- 45+: ~65%+
These are population averages, not personal forecasts.
After one miscarriage
After one miscarriage, the chance of miscarriage next time is often estimated around 10-25%. Recurrent losses are rarer, three consecutive miscarriages occur in about 1% of couples, and that’s when a structured workup for miscarriage causes is more often proposed.
Bleeding in early pregnancy: common, but worth checking
Bleeding in the first trimester is frequent (around 25%). It does not automatically mean miscarriage.
Seek medical advice promptly if you have:
- Heavy bleeding (soaking pads quickly)
- Worsening or severe pelvic pain
- Faintness, dizziness, shoulder-tip pain
- Fever or chills
- Foul-smelling discharge
Two tools guide decisions: ultrasound and serial beta-hCG.
Miscarriage causes vs risk factors: what you can change (and what you cannot)
A cause directly stops development (for example, a major chromosomal error). A risk factor raises the probability without guaranteeing a loss.
Non-modifiable factors include age and many random genetic events.
Modifiable factors can include smoking, alcohol, illicit drugs, uncontrolled diabetes or thyroid disease, some medication exposures, and some uterine/cervical problems.
Why is a single explanation sometimes missing? Early miscarriage causes can occur at embryo level and leave few traces in standard testing.
Chromosomal and genetic miscarriage causes (most common in the first trimester)
The leading category of miscarriage causes is genetic.
Aneuploidy
Aneuploidy means the embryo has too many or too few chromosomes (trisomy, monosomy, triploidy). These errors usually happen by chance during egg or sperm formation, or in the first cell divisions.
When tissue is available for testing, chromosomal abnormalities are found in roughly 50-60% of first-trimester miscarriages.
Inherited vs random findings
Most chromosomal findings are random. A smaller portion involves a parent carrying a balanced chromosomal rearrangement (balanced translocation). The parent is typically healthy, but embryos can inherit an unbalanced set. In recurrent loss, genetic counseling may be offered.
Implantation and early placental development problems
Another cluster of miscarriage causes involves implantation and the placenta.
- Implantation: the embryo must attach to a receptive endometrium (uterine lining). If the timing or signaling is off, loss can be extremely early.
- Early placental dysfunction: placental cells should remodel maternal blood vessels, if this invasion/remodeling is impaired, growth can stop.
If pregnancy tissue is available, products of conception testing may identify chromosomal issues. Placental pathology can sometimes show inflammation or vascular/clotting patterns that guide care later.
Maternal health conditions that can contribute
Sometimes the embryo is genetically typical, but the maternal environment disrupts early development.
Progesterone, luteal phase questions, and PCOS
Progesterone supports the endometrium. Low progesterone can be a sign of a failing pregnancy rather than the initial trigger.
“Luteal phase defect” remains debated because there is no consistently reliable test.
In selected situations (for example early bleeding or some recurrent-loss profiles), clinicians may discuss progesterone supplementation. In IVF, luteal support is routine.
PCOS (polycystic ovary syndrome) may be associated with miscarriage risk, often through insulin resistance and metabolic health.
Thyroid disease
Poorly controlled hypothyroidism or hyperthyroidism is linked to miscarriage risk. Thyroid antibodies may also be associated with risk in some studies, even when hormone levels are normal – one reason early pregnancy blood checks are common.
Uncontrolled diabetes
High glucose around conception and early pregnancy increases risk through effects on embryo development, blood vessels, and the placenta. Preconception optimization can reduce risk.
Autoimmune disease and antiphospholipid syndrome (APS)
A treatable cause in recurrent loss is APS, which increases clotting in placental vessels.
Testing usually includes:
- Anticardiolipin antibodies (IgG/IgM)
- Anti-beta-2 glycoprotein I antibodies (IgG/IgM)
- Lupus anticoagulant
Diagnosis requires persistent positivity on repeat testing (often 12 weeks apart). Treatment plans may include low-dose aspirin and heparin.
Uterus and cervix factors (structural miscarriage causes)
- Congenital uterine anomalies: septate, bicornuate, unicornuate, didelphys. A septum can reduce blood supply at the implantation site. Diagnosis may involve transvaginal ultrasound (sometimes 3D), MRI, or HSG, hysteroscopy can confirm and sometimes treat.
- Fibroids and polyps: risk is higher when lesions distort the uterine cavity (especially submucosal fibroids).
- Intrauterine adhesions (Asherman syndrome): can impair implantation, often after uterine procedures.
Cervical insufficiency (later losses)
With painless cervical dilation in the second trimester, clinicians consider cervical insufficiency. Cervical length can be measured by transvaginal ultrasound. In selected cases, progesterone and/or cerclage can reduce recurrence.
Infections and fever
Many everyday viral infections do not cause miscarriage. Still, some infections and high fever can contribute through inflammation or placental infection.
Depending on symptoms/exposure, clinicians may consider toxoplasmosis, rubella, CMV, herpes simplex, parvovirus B19, listeria, syphilis, or bacterial vaginosis. Testing is usually targeted.
Lifestyle, exposures, and medications
Some miscarriage causes are linked to exposures that can be changed.
- Smoking (including secondhand smoke) increases miscarriage risk.
- Alcohol is associated with miscarriage risk, most guidance supports avoiding it while trying to conceive and during pregnancy.
- Illicit drugs (notably cocaine) increase complications, cannabis is also associated with adverse outcomes.
Practical supports:
- Folic acid before conception
- Limit caffeine to about 200 mg/day
- Avoid overheating (hot tubs/saunas) in early pregnancy
- Review workplace/home exposures (solvents, pesticides) early
Medication review matters. Some drugs are incompatible with pregnancy (for example methotrexate, warfarin). Some NSAIDs may interfere with implantation biology, discuss use with a clinician.
Paternal factors
Paternal age shows a modest association with miscarriage risk in some studies. Higher sperm DNA fragmentation has also been linked to poorer embryo development, particularly in assisted reproduction, and may be discussed in recurrent loss.
What usually does not cause miscarriage (common worries)
Evidence does not support moderate exercise, sex in an uncomplicated pregnancy, flying, spicy food, or diagnostic ultrasound as typical miscarriage causes.
What miscarriage causes can mean for a future pregnancy
Some contributors can be treated or risk can be reduced: optimizing thyroid/diabetes control, treating APS, addressing a uterine septum or cavity-distorting fibroids/polyps, managing cervical insufficiency, stopping smoking, and reviewing medications.
Other times, miscarriage causes are random chromosomal events that could not have been prevented.
Key takeaways
- Miscarriage causes in the first trimester are most often chromosomal abnormalities occurring by chance.
- Bleeding is common, ultrasound and serial beta-hCG clarify whether a pregnancy is progressing, failing, or ectopic.
- Most early losses are not caused by a single action or moment.
- Some risk factors are modifiable (smoking, alcohol/drugs, medication exposures, uncontrolled thyroid/diabetes, some uterine issues).
- Recurrent loss deserves a structured evaluation for miscarriage causes, including uterine anatomy, thyroid/metabolic checks, and APS testing.
- Professional support exists through your GP, midwife, OB-GYN, or early pregnancy unit. For personalized guidance and free child health questionnaires, you can also download the Heloa app.
Questions Parents Ask
Can stress cause a miscarriage?
Rassurez-vous: day-to-day stress, an argument, or a fright is not considered a typical cause of first‑trimester miscarriage. Many losses happen because the embryo cannot develop normally (often due to chromosomal changes) and that isn’t something a parent “triggers.” If stress feels overwhelming, gentle support can still be helpful—for your sleep, appetite, and overall wellbeing—without framing it as your fault.
Can sex cause a miscarriage?
In an uncomplicated pregnancy, sex does not cause miscarriage. Light spotting after sex can happen because the cervix is more sensitive in early pregnancy, and it can look alarming while still being harmless. If bleeding becomes heavy, pain increases, or you feel unwell, checking in with a clinician can bring clarity and reassurance.
What tests can help find the cause after a miscarriage?
After a single early loss, extensive testing is not always proposed, because many causes are random and won’t show up on routine exams. When losses repeat, options may include: genetic testing of pregnancy tissue (if available), blood tests for thyroid function and antiphospholipid syndrome, and imaging to look at the uterus (e.g., ultrasound/3D ultrasound, sometimes MRI or HSG). These steps can identify treatable factors and support the next pregnancy with a clear plan.
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