Many parents look up miscarriage causes late at night, replaying every detail: the bumpy auto ride, office stress, a cup of tea, that short walk. Was it something I did?
In most first-trimester losses, miscarriage causes are not daily actions. They are usually chromosome changes in the embryo, early placental development that does not progress, or health conditions that affect implantation and blood flow.
What a miscarriage is (including very early “chemical” loss)
A miscarriage (early pregnancy loss) is the spontaneous end of a pregnancy before viability outside the uterus. Many clinicians use 20 weeks, some settings use 24 weeks.
A very early loss can happen before anything is visible on ultrasound. This is a chemical pregnancy: the test is positive because beta-hCG rises briefly, then falls, and bleeding starts—often around the expected period.
Common terms:
- Blighted ovum (anembryonic pregnancy): gestational sac seen, no embryo visible.
- Missed miscarriage: development stopped, but bleeding/cramps may not start right away.
- Threatened miscarriage: bleeding/cramps with a closed cervix, pregnancy may still continue.
Most early miscarriage causes are not sex, moderate exercise, commuting, or one stressful day.
Miscarriage vs stillbirth, ectopic pregnancy, and molar pregnancy
- Miscarriage: early loss.
- Stillbirth: loss later in pregnancy (often from 24 weeks onward, definitions vary).
- Ectopic pregnancy: implantation outside the uterus (usually a tube). It cannot continue and may rupture.
- Molar pregnancy: abnormal placental tissue growth, hCG may be unusually high and ultrasound has a typical appearance.
When miscarriages most often happen
Most miscarriages occur in the first trimester (up to 12–13 weeks). Second-trimester losses are less common, later loss makes clinicians think more about placenta, infection, uterus/cervix, and maternal conditions—different miscarriage causes than very early loss.
How common miscarriage is (and what risk can look like)
About 10–20% of known pregnancies end in miscarriage (true numbers are higher because many chemical pregnancies go unrecorded). Around 80% occur by 13 weeks. Second trimester (13–19 weeks) risk is often quoted around 1–5%.
Age and risk
Risk rises with maternal age due to more frequent chromosome errors in eggs:
- 20s: ~12–15%
- 35–39: ~18%
- 40–44: ~37–40%
- 45+: ~65%+
These are averages, not individual predictions.
After one miscarriage
After one miscarriage, many people have a healthy ongoing pregnancy next time. Risk of miscarriage in the next pregnancy is often quoted around 10–25%. If losses repeat, doctors are more likely to evaluate treatable miscarriage causes.
Bleeding in early pregnancy: common, but worth checking
Bleeding in the first trimester is common (around 25%). Light brown spotting without pain can happen, but bleeding still deserves assessment.
Seek urgent care if you have:
- Heavy bleeding (soaking a pad quickly)
- Severe or worsening pelvic pain, or one-sided pain
- Dizziness, fainting, shoulder-tip pain
- Fever or chills
- Foul-smelling discharge
Key tools are ultrasound and serial beta-hCG.
Miscarriage causes vs risk factors (what you can and can’t change)
A cause directly stops development (for example, a major chromosome error). A risk factor increases the chance without guaranteeing miscarriage (for example, smoking).
Non-modifiable:
- Age
- Many random genetic events at conception
Modifiable:
- Smoking/tobacco chewing and second-hand smoke
- Alcohol and illicit drugs
- Uncontrolled diabetes or thyroid disease
- Certain medication exposures
- Some uterine/cervical problems that can be treated
Thinking “what can I influence now?” helps, without turning miscarriage causes into self-blame.
Chromosomal and genetic miscarriage causes (most common)
The commonest first-trimester explanation is genetic.
Aneuploidy means too many/too few chromosomes (trisomy, monosomy, triploidy), usually by chance during egg/sperm formation or early cell divisions. When tissue can be tested, abnormalities are found in about 50–60% of first-trimester miscarriages.
Most findings are random. A smaller proportion involves a parent with a balanced translocation, genetic counselling is often offered when losses repeat.
Implantation and early placental development problems
Some miscarriage causes involve implantation and placenta:
- The embryo must attach to a receptive endometrium (uterine lining).
- Placental cells must connect to maternal blood supply and remodel blood vessels.
If pregnancy tissue is available, products of conception testing may identify chromosomal causes. Placental microscopy can sometimes suggest inflammation or vascular/clotting patterns.
Maternal health conditions that can contribute
Progesterone, luteal phase questions, and PCOS
Progesterone supports the uterine lining. Low levels can be seen when a pregnancy is failing, but may not be the original trigger. “Luteal phase defect” remains debated.
In selected situations (for example early bleeding or some recurrent-loss profiles), clinicians may discuss progesterone supplementation. In IVF, luteal support is routine.
PCOS can be associated with miscarriage risk, often linked with insulin resistance and metabolic health.
Thyroid disease
Untreated or poorly controlled hypothyroidism/hyperthyroidism is linked to higher miscarriage risk. Thyroid antibodies may also be associated in some studies, so early testing and dose adjustment are common.
Uncontrolled diabetes
Poorly controlled diabetes around conception can increase miscarriage risk through effects on embryo development, blood vessels, and placental formation.
Autoimmune disease and antiphospholipid syndrome (APS)
A key treatable contributor in recurrent loss is APS, which raises clotting risk in placental vessels.
APS testing typically 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). Care is specialist-led and may include low-dose aspirin and heparin.
Uterus and cervix factors
- Congenital uterine anomalies (septate, bicornuate, unicornuate, didelphys) can affect implantation, diagnosis may involve transvaginal (sometimes 3D) ultrasound, MRI, or HSG. A septum can often be corrected by hysteroscopy.
- Fibroids and polyps: risk is higher when they distort the uterine cavity (especially submucosal fibroids).
- Intrauterine adhesions (Asherman syndrome): can affect implantation, often assessed with ultrasound and hysteroscopy.
For second-trimester painless dilation, clinicians consider cervical insufficiency, cervical length on transvaginal scan guides options like progesterone and/or cerclage.
Infections and fever
Many common viral illnesses do not cause miscarriage, but high fever in early pregnancy should be assessed. Depending on symptoms/exposure, doctors may consider toxoplasmosis, rubella, CMV, herpes, parvovirus B19, listeria, syphilis, or bacterial vaginosis (usually symptom-led testing).
Lifestyle, exposures, and medicines
Some miscarriage causes overlap with modifiable exposures:
- Tobacco (including second-hand smoke)
- Alcohol
- Illicit drugs (cocaine is high-risk, cannabis is also linked to adverse outcomes)
Useful basics:
- Folic acid before conception
- Limit caffeine to about 200 mg/day
- Avoid overheating (hot tubs/sauna) in early pregnancy
- Review workplace/home exposures (solvents, pesticides)
- Review medicines early (for example isotretinoin, methotrexate, warfarin are unsafe)
How doctors assess miscarriage
Symptoms may include bleeding, cramps, clots/tissue—or no symptoms at all.
Early ultrasound (often transvaginal) checks sac, embryo, heartbeat, and dating. Serial beta-hCG trends help: rising is expected in early viable pregnancy, plateauing/falling suggests a non-developing pregnancy, interpreted with scan findings.
After a single early miscarriage, extensive testing is not routine. In recurrent loss, evaluation may include uterine imaging, thyroid/metabolic tests (TSH, HbA1c), APS testing, and genetic testing in selected cases.
What usually doesn’t cause miscarriage
In an uncomplicated pregnancy, evidence does not show that moderate exercise, sex, air travel, spicy food, or diagnostic ultrasound are typical miscarriage causes.
What miscarriage causes can mean for a future pregnancy
Some contributors can be treated or risk reduced: better diabetes/thyroid control, APS treatment, correcting a uterine septum, managing cavity-distorting fibroids/polyps, addressing cervical insufficiency, stopping tobacco, and reviewing medicines/exposures.
Other times, miscarriage causes are random chromosomal events—nothing could have prevented them.
Key takeaways
- The most common miscarriage causes in the first trimester are random chromosomal abnormalities.
- Bleeding is common, ultrasound and serial beta-hCG clarify the situation.
- Heavy bleeding, severe pain, dizziness/fainting, fever/chills, or foul-smelling discharge needs prompt medical assessment.
- Some factors are modifiable (tobacco, alcohol/drugs, uncontrolled diabetes/thyroid disease, certain uterine issues), others are not (age, many genetic events).
- If losses repeat, your doctor may look for treatable miscarriage causes and plan closer follow-up.
- Support is available through your obstetrician/gynaecologist. You can also download the Heloa app for personalised advice and free child health questionnaires.
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