Pregnancy can be as much a time of anxiety as joyous anticipation. In our age of increased access to information, more and more of us become aware of the sad truth that not every positive pregnancy test ends with a baby, and it's natural to want numbers for reassurance.
But if you're searching online for information about your risk of miscarriage, you're probably more confused than reassured. Everyone has a different statistic for the risk of miscarriage. Here's some help in sorting out the common claims about miscarriage statistics.
This statistic is an estimate for the percentage of fertilized eggs that do not go on to result in a full-term pregnancy, factoring in miscarriages but also failed implantations that usually pass without the mother ever missing a period. If you already know you're pregnant, this is not a number you should be worrying about.
Because there's only around a 30% chance of getting pregnant each menstrual cycle even when you're having regular sex, researchers speculate that fertilized eggs often fail to implant, usually with the woman unaware that conception occurred. Laboratory studies on IVF patients have found that a very large percentage of eggs harbor chromosome abnormalities (the leading cause of miscarriage), and another study found that in natural cycles, about 22% of all conceptions never complete implantation. Considering such evidence, some scientists have speculated that if you factor in fertilized eggs that don't implant along with pregnancies that end in miscarriage, around 70-75% of conceptions end up miscarrying.
But whether these failed implantations can be defined as a "miscarriage" is a matter of opinion. Most doctors consider pregnancy to begin at implantation rather than at fertilization. At the very least, by the time implantation has occurred and the hCG hormone is detectable in the blood and urine (meaning, by the time you know you're pregnant), it's safe to say this statistic is irrelevant.
About 30% of pregnancies end in miscarriage.
In the same study that found 22% of conceptions fail to implant, it was also found that 31% of pregnancies confirmed after implantation end in miscarriage.
That would mean that about 1 in 3 pregnancies miscarry, but before you worry too much about that, note that this number came from a study population of women who were confirmed to be pregnant at the very earliest point that it's scientifically possible to detect a pregnancy. In real life, most women find out they are pregnant at a later point than the participants in this study, and the risk of miscarriage drops as pregnancy progresses. That would mean that most women have a lower risk of miscarriage by the time they confirm their pregnancies.
Still, it's worth pointing out that many experts consider this statistic as an argument against taking ultra-sensitive early pregnancy tests before missing a menstrual period. Use of such pregnancy tests increases the risk of detecting a transient, nonviable pregnancy that will miscarry within a few days and otherwise would have just looked like a regular menstrual period. Knowing about such pregnancies can be a source of stress for many women, and if this is true for you, it's worth waiting to test until your period is actually late.
15-20% of confirmed pregnancies end in miscarriage.
For the general population of pregnant women, this is usually the most relevant statistic. About 15 to 20% of all women with a verified pregnancy will end up having a miscarriage. Since the general population includes women who weren't trying to get pregnant and might not have been tracking their menstrual periods, many are already a few weeks along -- possibly more than halfway through the first trimester -- by the time they confirm their pregnancies. As stated above, the further a pregnancy progresses, the lower the risk of miscarriage, so that's the reason for the disparity between this statistic and the one above.
If you're more than 5 weeks pregnant but still in the first trimester, this statistic is probably the most relevant one for you. But remember that your own personal risk might be higher or lower depending on several other factors, including your age, lifestyle, and past pregnancy history.
Most doctors agree that seeing a fetal heartbeat on ultrasound means the risk of miscarriage is much lower. Confirmation of a fetal heartbeat means that the baby has passed the initial stages of development wherein the majority of first trimester miscarriages occur.
It's hard to pick a specific number for the risk of miscarriage at this point. Some cite numbers like a 4-5% risk of miscarriage at this point, but studies show a widely varying risk based on individual factors. The one thing that's safe to say, however, is that seeing the baby's heartbeat is a good sign. It means the baby is growing as it should be, and there's little reason to be concerned at this point.
Unfortunately, there is one exception to this rule. A slow fetal heart rate (less than 100 beats per minute) may signal an impending miscarriage, though this isn't true in 100% of cases.
The majority of miscarriages occur in the first trimester.
Over 80% of miscarriages occur before 12 weeks, so the chances look good for a healthy baby once you've finished the first trimester. Again, many individual factors are in play, but if 15-20% of pregnancies end in miscarriage and 80% of miscarriages occur in the first trimester, a safe estimate would be that in the general population the risk of pregnancy loss after 12 weeks is 3-4%. After 20 weeks, when a loss would be termed a stillbirth rather than a miscarriage, the risk is around 1 in 160.
Patient information: Miscarriage. UpToDate. Accessed: 14 Apr 2010. http://www.utdol.com/patients/content/topic.do?topicKey=~OL0HfsnsRA4Z0
Stillbirth. March of Dimes. Accessed: 14 Apr 2010. http://www.marchofdimes.com/professionals/14332_1198.asp
Wilcox AJ, Weinberg CR, O'Connor JF, Baird DD, Schlatterer JP, Canfield RE, Armstrong EG, Nisula BC. "Incidence of early loss of pregnancy." N Engl J Med. 1988 Jul 28;319(4):189-94.