A miscarriage is the spontaneous loss of a pregnancy that occurs in the first 20 weeks of pregnancy. Miscarriages are distressing and common, affecting approximately one in five pregnancies. In most cases when pregnancy fails it is because of a problem with either the embryo (chromosomal) or developmental, or both. Miscarriage is almost never caused by anything the woman did or failed to do.
Interestingly, human beings are very inefficient reproducers. The majority of embryos that we make, whether in nature or the IVF lab, are not normal. Pregnancy is a very strong filter of the abnormal, so most abnormal embryos will fail to implant. More than half of all miscarriages happen because of an irregular number of chromosomes in the embryo and the pregnancy does not develop properly from the start. The miscarriage risk increases with increasing maternal age as older eggs are more likely to be abnormal.
Things that do not cause miscarriage
The list of things that do NOT cause miscarriage is far longer than those that do. Miscarriages in early pregnancy are not caused by:
- exercising, working, travelling, having sex, eating the “wrong” food, having a few drinks before you knew you were pregnant, morning sickness, or using birth control pills
- a fall, a blow or a fright
- stress or worry
- wondering whether or not you wanted the baby
- having had an abortion
Things that may increase the chances of miscarriage
The risk of miscarriage may be increased in pregnant women who:
- drink alcohol
- use illegal drugs
- are exposed to high levels of radiation or toxic agents
After an isolated spontaneous miscarriage, the chance of having a successful pregnancy in the future is quite high. Every time you get pregnant, you have the same age-based risk of pregnancy loss. Based on chance alone, the odds of having two miscarriages in a row are:
[age-based chance] X [age-based chance]
In other words, a 32-year-old woman who has a 15% chance of miscarrying in her first pregnancy will have a 15% of 15%, or a 2.25% chance of having a second consecutive miscarriage, based on chance alone.
In reality, however, the chance of consecutive miscarriages is higher than one would expect based on chance alone. That is because there are real and persistent causes of pregnancy loss that add to the chance of having a miscarriage.
When to take investigations further?
While pregnancy loss usually is a one-time occurrence, up to one in twenty couples experience two miscarriages in a row, and one in one hundred has three or more. In some cases, these couples have an underlying problem that is causing the losses. Most women’s health organisations suggest that you should have investigations after two consecutive miscarriages, and all agree they should be done after three.
Most cases of recurrent miscarriage (two or more) will remain unexplained even after detailed investigations have been performed. It is important to understand that although researchers are constantly trying to find new causes for miscarriage that there will always be some couples whose pregnancy losses remain unexplained. Importantly, the prognosis for future successful pregnancy in the unexplained group is usually better than it is for couples in which a recognised cause is identified.
According to the European Society of Human Reproduction and Embryology (ESHRE), these are the tests with good evidence to support them, to help try and determine a cause for the miscarriage:
- Blood tests for:
o Parental chromosomes to rule out a structural rearrangement
o Anticardiolipin antibodies
o Thyroid function
- A three-dimensional (3D) ultrasound to rule out a septum or fibroid that might be preventing normal implantation.
We no longer test for genetic blood clotting disorders, and the role of ‘natural killer cell’ testing has been called into question in recent years.
Testing sperm for DNA fragmentation is controversial, as is a routine sampling of the lining of the uterus.
Care in future pregnancies
Sometimes the problem that caused the miscarriages can be treated. Surgery may help some problems of the uterus and cervix. In other cases, hormone or anti-clotting treatment may help.
If chromosomal problems are found in the parents, your doctor may advise genetic counselling. A genetic counsellor can help you and your partner learn what risks a genetic problem might pose for future pregnancies. In the past, couples had to wait until the pregnancy was established until the fetus could be tested for chromosomal problems by amniocentesis or chorionic villus sampling. It is now possible to undergo an IVF cycle and test the embryos before they are implanted.
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