The causes of miscarriage

If a mis­car­riage hap­pens dur­ing the first tri­mester of preg­nancy (the first three months), it is usu­ally due to prob­lems with the foetus (the unborn baby). If a mis­car­riage hap­pens dur­ing the second tri­mester of preg­nancy (between weeks 14 and 26), it is usu­ally the res­ult of an under­ly­ing health con­di­tion in the mother.

First tri­mester miscarriages 

Most first tri­mester mis­car­riages are caused by prob­lems with the chro­mo­somes of the foetus.  Chro­mo­somes are blocks of DNA. They con­tain a detailed set of instruc­tions that con­trol a wide range of factors, from how the cells of the body develop to what col­our eyes a baby will have.

For a preg­nancy to be suc­cess­ful, a foetus needs 46 chro­mo­somes: 23 are from the father’s sperm and 23 are from the mother’s egg.

Some­times, some­thing can go wrong at the point of con­cep­tion and the foetus receives too many or not enough chro­mo­somes. The reas­ons for this are often unclear, but it means that the foetus will not be able to develop nor­mally, res­ult­ing in a miscarriage.

Risk factors

Prob­lems with chro­mo­somes often hap­pen by chance. However, a num­ber of known risk factors increase the chances of these prob­lems occurring.

Age

The most import­ant risk factor for mis­car­riage is prob­ably the age of the mother:

• Women under 25 have a 9% risk of hav­ing a miscarriage.
• Women between 25 and 29 have an 11% risk of hav­ing a miscarriage.
• Women between 30 and 34 have a 15% risk of hav­ing a miscarriage.
• Women between 35 and 39 have a 25% risk of hav­ing a miscarriage.
• Women between 40 and 44 have a 51% risk of hav­ing a miscarriage.
• Women over 45 years have a 75% risk of hav­ing a miscarriage.

Other risk factors

Other risk factors for hav­ing a mis­car­riage include:

• obesity,
• smoking dur­ing pregnancy,
• drug mis­use dur­ing preg­nancy (par­tic­u­larly cocaine),
• drink­ing more than 200mg of caf­feine a day (equi­val­ent of two mugs of tea or instant cof­fee, one mug of fil­ter cof­fee or five cans of cola), and
• drink­ing more than two units of alco­hol a week (one unit is half a pint of bit­ter or ordin­ary strength lager, a small glass of wine or a 25ml meas­ure of spirits).

Second tri­mester miscarriages

Chronic health conditions

There are a num­ber of chronic (long-lasting) health con­di­tions that can increase the risk of hav­ing a mis­car­riage. These are:

• dia­betes (if it is poorly controlled),
• severe high blood pres­sure (hypertension),
• lupus (a con­di­tion where the immune sys­tem attacks healthy tissue),
• kid­ney dis­ease, and
• an over­act­ive or under­act­ive thyroid gland.

Infec­tions

There are a num­ber of infec­tions that may increase the risk of hav­ing a mis­car­riage. These include:

• rubella (Ger­man measles),
• cyto­meg­a­lovirus (CMV), and
• tox­o­plas­mosis (a bac­terial infection).

Mis­car­riage can also be caused by a bac­terial infec­tion of the vagina. This type of infec­tion is known as bac­terial vagin­osis (BV).

Anti­bod­ies

Anti­bod­ies are pro­teins that are pro­duced by the immune sys­tem to fight infec­tion.  Approx­im­ately 15% of women with a his­tory of recur­rent mis­car­riages (three or more mis­car­riages in a row) have a higher than usual level of an anti­body called anti­phos­phol­ipid (aPL) in the blood. The aPL anti­bod­ies are known to cause blood clots. These blood clots can block the sup­ply of blood to the foetus, which can cause a mis­car­riage. Hav­ing a high num­ber of aPL anti­bod­ies in your blood is known as Hughes syn­drome. See Use­ful links for more inform­a­tion about Hughes syndrome.

Womb struc­ture

Prob­lems and abnor­mal­it­ies with the womb can also lead to second tri­mester mis­car­riages. Pos­sible prob­lems with the struc­ture of the womb include:

• fibroids (non-cancerous growths in the womb), and
• scar­ring on the sur­face of the womb.

Weakened cer­vix

In some cases, the muscles of the cer­vix (the open­ing of the womb) are weaker than usual. This is known as a weakened cer­vix. The muscle weak­ness can cause the cer­vix to open too early dur­ing preg­nancy, lead­ing to a miscarriage.

Hyper­pro­lactin­aemia

Pro­lactin is a hor­mone that is pro­duced dur­ing pregnancy. It helps pre­pare the breasts for breastfeeding. Sometimes, women have a higher level of pro­lactin in their body than usual. This is known as hyper­pro­lactin­aemia. Some lim­ited evid­ence sug­gests that hyper­pro­lactin­aemia may be linked to an increased risk of miscarriage.

Poly­cystic ovary syndrome

Poly­cystic ovary syn­drome (POS) is a con­di­tion where the ovar­ies are lar­ger than nor­mal. It can lead to hor­monal imbal­ances inside the womb. POS is known to be a lead­ing cause of infer­til­ity. There is some evid­ence to sug­gest that it may also be linked to an increased risk of mis­car­riage in women who are still fer­tile. How­ever, the exact role that POS plays in mis­car­riages is unclear.

Mis­con­cep­tions about miscarriage

There are a num­ber of widely held assump­tions about the pos­sible causes of mis­car­riages. For example, mater­nal stress is often believed to be a cause. How­ever, there is no evid­ence to sup­port such claims.

An increased risk of mis­car­riage is also not linked to:

• a mother’s emo­tional state dur­ing preg­nancy, such as being stressed or depressed,
• hav­ing a shock or fright dur­ing pregnancy,
• exer­cise dur­ing preg­nancy (but dis­cuss what type of exer­cise is suit­able for you dur­ing preg­nancy with your GP or midwife),
• lift­ing or strain­ing dur­ing pregnancy,
• work­ing dur­ing preg­nancy, and
• hav­ing sex dur­ing pregnancy.

Dia­gnos­ing miscarriage 

A num­ber of tests can con­firm whether your preg­nancy has ended and you have had a mis­car­riage. The tests can also con­firm whether:

• you have had an incom­plete mis­car­riage, where there is still some feotal tis­sue left in your womb, or
• you have had a com­plete mis­car­riage, where all the foetal tis­sue has been passed out of your womb.

The tests include:

• blood and urine tests, which can be used to meas­ure a hor­mone asso­ci­ated with preg­nancy called beta hCG,
• ultra­sound, and
• a pel­vic examination.

Recur­rent miscarriages

If you have had recur­rent mis­car­riages (three or more mis­car­riages in a row), fur­ther tests can check if there is an under­ly­ing cause. How­ever, some of these tests can only be used if you become preg­nant again. These fur­ther tests are out­lined below.

Karyotyp­ing

If you have had recur­rent mis­car­riages, you and your part­ner can be tested for abnor­mal­it­ies in your chro­mo­somes that could be caus­ing the prob­lem. This is known as karyotyping.

If karyotyp­ing detects prob­lems with your or your partner’s chro­mo­somes, you can be referred to a clin­ical genet­i­cist (gene expert). They will be able to explain what the chances of you hav­ing a suc­cess­ful preg­nancy in the future are and whether there are any fer­til­ity treat­ments, such as IVF, that you could try. This type of advice is known as genetic counselling.

Pel­vic ultrasound

A pel­vic ultra­sound can be used to check the struc­ture of your womb for any abnor­mal­it­ies. The pro­ced­ure involves using an ultra­sound scan­ner to study your lower abdo­men and pelvis.

Vaginal ultra­sound

A vaginal ultra­sound can check if you have a weakened cer­vix. This test can usu­ally only be car­ried out if you become preg­nant again.

A vaginal ultra­sound is sim­ilar to a pel­vic ultra­sound but in this pro­ced­ure a small piece of equip­ment, known as a trans­ducer, is inser­ted into your vagina. This pro­duces more detailed scan of your cer­vix. The pro­ced­ure can feel a little uncom­fort­able but is not painful.

Blood test­ing

Your blood can be checked to see whether you have high levels of the hor­mone pro­lactin and/or the anti­phos­phol­ipid (aPL) antibody. Testing for pro­lactin can only be done if you become preg­nant again.