Understanding miscarriage
Why do miscarriages happen?
Why early miscarriages happen, how age and some medical conditions may matter, and why a miscarriage is usually not something you caused.
Miscarriage is common – sadly, far more common than you think. But whether you know that or not, it hurts you as much....
One of the first questions many women ask is: “Why did this happen?”
Sometimes there is an identifiable reason. Most often there is not. And in many early miscarriages, the explanation lies in the pregnancy itself.
An embryo needs the correct set of chromosomes to continue developing normally. Sometimes an egg or sperm contributes an incorrect number, or an error occurs around fertilisation and early cell division.
When this happens, the pregnancy may stop developing very early.
This is one of the commonest explanations for early miscarriage, and the chance of chromosomal problems increases as the age of the egg increases.
As women get older, a greater proportion of eggs are chromosomally abnormal. That means fewer eggs are capable of producing a chromosomally normal embryo.
This is why miscarriage becomes more common with increasing maternal age.
Age changes probability; it does not tell us what will happen in an individual pregnancy.
Differences in the shape of the uterine cavity can be associated with recurrent miscarriage. Fibroids that significantly distort the cavity, uterine septa and some other abnormalities may be relevant in particular women.
A pelvic ultrasound is often the starting point. Depending on the history and findings, more detailed imaging may occasionally be appropriate.
But finding a fibroid or another scan finding does not automatically mean that it caused the miscarriage.
Thyroid problems can be relevant. Diabetes may matter when the history suggests it. Other hormonal investigations may be appropriate if there are symptoms suggesting conditions such as PCOS or Prolactin disturbance.
Another important condition is antiphospholipid syndrome (APS), an acquired condition affecting blood clotting that is associated with recurrent miscarriage. The important point is not to order every blood test available. Investigations should answer a sensible clinical question.
Ordinary activity, work, exercise and sex do not usually cause miscarriage. Lifestyle and general health can influence pregnancy risk — smoking, excessive alcohol, very high caffeine intake and being significantly under- or overweight are among the factors associated with miscarriage risk — but that is very different from saying a particular action caused an individual miscarriage. If you miscarried after exercising, travelling, working a long day or having sex, the timing does not mean that activity caused it.
Even after appropriate investigations, no definite cause is found in many couples with recurrent miscarriage. That can be frustrating, but “unexplained” does not mean there is no chance of a successful pregnancy. For many women, particularly at younger ages, the outlook for a future successful pregnancy remains good even when investigations do not identify a specific cause.
One miscarriage is unfortunately common. Repeated losses deserve a different conversation. UK guidance traditionally defines recurrent miscarriage as three or more first-trimester miscarriages, but doctors are encouraged to consider investigation after two when the history suggests the losses may not simply be sporadic. Other international guidance uses two or more pregnancy losses. So the practical question is not simply “Have I reached three?” It is “Does my history give us a reason to start looking now?”
Sources and further reading
- RCOG Green-top Guideline No. 17 and patient information on recurrent miscarriage
- ESHRE Guideline on Recurrent Pregnancy Loss
Medical review
Medically reviewed by Dr Mahadeo Bhide, MD, MRCOG
Last medically reviewed: September 2026