Obstetrician and Gynaecologist
What is the most common cause of miscarriage?
Spontaneous miscarriage occurs in 10–20% of clinically recognised pregnancies, the major underlying cause being embryonic aneuploidy; this means the presence of an abnormal number of chromosomes in a cell e.g. a human cell having 45 or 47 chromosomes instead of the usual 46. The risk of these chromosomal problems increases significantly with age.
What causes recurrent miscarriage?
Recurrent miscarriage is usually defined as having three or more miscarriages in a row, whether or not you have had any healthy pregnancies, and affects approximately 1% of couples.
There are a number of things which may play a part in recurrent miscarriage but it is a complicated problem and more research is still needed:
The older you are, the greater your risk of having a miscarriage.
The more miscarriages you have had already, the more likely you may be to have another one.
For around three to five in every 100 women who have recurrent miscarriages, they or their partner have an abnormality on one of their chromosomes (the genetic structures within our cells that contain our DNA and the features we inherit from our parents). Although such abnormalities may cause no problem for you or your partner, they may sometimes cause problems if passed on to your baby.
It is not clear how far major irregularities in the structure of your womb can affect the risk of recurrent miscarriages. Estimates of the number of women with recurrent miscarriage who also have these irregularities range from two out of 100 to as many as 37 out of 100. Women who have serious anatomical abnormalities and do not have treatment for them seem to be more likely to miscarry or give birth early. Minor variations in the structure of your womb do not cause miscarriages.
In some women, the entrance of the womb (the cervix) opens too early in the pregnancy and causes a miscarriage in the third to sixth month. This is known as having a weak (or ‘incompetent’) cervix. It is overestimated as a cause of miscarriage because there is no really reliable test for it outside of pregnancy.
Some people have suggested that some women miscarry because their immune system does not respond to the pregnancy in the usual way. This is known as an alloimmune reaction. There is currently no clear evidence to support this theory.
Can medical conditions increase the risk of recurrent miscarriage?
Antiphospholipid syndrome (also known as Hughes Syndrome), remains the most important treatable cause of recurrent miscarriage, and all women with recurrent pregnancy loss should be tested for this condition.
Around 15 in every 100 women who have had recurrent miscarriages have particular antibodies, called antiphospholipid antibodies (aPL), in their blood; fewer than two in every 100 women with normal pregnancies have aPL antibodies. Some people produce antibodies that react against the body’s own tissues; this is known as an autoimmune response and this is what happens to women who have aPL antibodies. If you have aPL antibodies and a history of recurrent miscarriage, your chances of a successful pregnancy may be reduced.
Diabetes or thyroid disorders
Diabetes or thyroid disorders can be factors in single miscarriages. They do not cause recurrent miscarriage as long as they are treated and well-controlled.
Prolactin is a hormone which prepares a pregnant woman’s breasts to produce milk. When a woman produces too much prolactin, this is known as hyperprolactinaemia. It is not yet clear whether this condition plays a role in recurrent miscarriage because the evidence is conflicting.
If you have polycystic ovaries, your ovaries are slightly larger than normal ovaries and produce more small follicles than normal. This may be linked to an imbalance of hormones. Just under half of women with recurrent early miscarriages have polycystic ovaries; this is about twice the number of women in the general population. Having polycystic ovaries is not a direct cause of recurrent miscarriage and it does not mean that you are at any greater risk of further miscarriages. We are still not sure what the link is. Many women with polycystic ovaries and recurrent miscarriage have high levels of a hormone called luteinising hormone (LH) in their blood. Reducing the level of LH before pregnancy, however, does not improve your chances of a successful birth.
Certain inherited conditions may mean that your blood may be more likely to clot than is usual. These conditions are known as thrombophilias. Although thrombophilia has been thought to play some part in miscarriage, we do not yet know enough about how or why that is.
What treatment is available for recurrent miscarriage?
Women who have supportive care from the beginning of a pregnancy have a better chance of a successful birth. There is some evidence that attending an early pregnancy clinic (EPC) can reduce the risk of further miscarriages.
There is evidence that if you have aPL antibodies and a history of recurrent miscarriages, treatment with low-dose aspirin tablets and low-dose heparin injections in the early part of your pregnancy may improve your chances of a live birth.
Progesterone supplements (one of the natural hormones that support pregnancy) may be of some help in supporting pregnancies in the first trimester, although there is conflicting evidence in terms of its effectiveness. Your doctor will usually discuss this with you and, based on your history, may recommend it in some cases.
Steroids (certain sorts of natural or synthetic hormones) have been used to treat aPL antibodies in recurrent miscarriage, but they do not seem to improve the chances of successful delivery and they carry significant risks for you and your baby, compared with aspirin and heparin.
Although you may have a higher risk of miscarriage if you have an inherited tendency to blood clotting (thrombophilia), you may still have a healthy and successful pregnancy. At present, there is no test available to identify whether you will miscarry if you have thrombophilia. You may, though, be offered treatment to reduce the risk of a blood clot.
If you have a history of recurrent miscarriage and you lose your next baby, we may suggest checking for abnormalities in the embryo or the placenta afterwards. We do this by checking the chromosomes of the embryo through karyotyping, although it is not always possible to get a result. They may also examine the placenta through a microscope. The results of these tests may help to identify and discuss with you your possible choices and treatment.
If you’ve had miscarriages in the fourth to sixth month of pregnancy, or if you have a history of going into labour prematurely, you may be offered tests (and treatment if necessary) for an infection known as bacterial vaginosis (BV). If you have BV, treatment with antibiotics may help to reduce the risks of losing your baby or of premature birth. There is not enough evidence to be sure that it makes any difference to the chances of a baby surviving.
A pelvic ultrasound scan (2D or 3D) will be performed to check for and assess any abnormalities in the structure of your womb so that they can be treated if necessary. Another method of screening using hysterosalpingography (an X-ray of the fallopian tubes using fluid injected through the entrance of the womb) has no advantages over pelvic ultrasound and causes more discomfort, so it is not usually necessary. If an abnormality of the uterus is suspected, a procedure known as a hysteroscopy can be carried out. This is usually performed under general anaesthetic and any abnormalities can often be addressed at that stage.
Treatment to prevent or change the response of the immune system (known as immunotherapy) is not recommended for women with recurrent miscarriage. It has not been proven to work, does not improve the chances of a live birth and it may carry serious risks (including transfusion reaction, allergic shock and hepatitis).
How common is it that a cause for repeated miscarriages cannot be identified?
Regrettably, in approximately 50% of couples investigated for recurrent pregnancy loss, no identifiable cause is found. However, in couples where no diagnosis is made, the chances of a subsequent successful pregnancy are good.