No one really knows how often human pregnancies are lost. Investigations of the rates of loss are inherently difficult due to the inaccessibility of information.
- Many pregnancies are lost at home without the aid of medical facilities. These losses may go unreported.
- In the USA, there is no formal reporting of pre-viable pregnancy losses to a centralized agency (as there is with live birth statistics).
Most of the research on incidence rates of pregnancy loss incorporates their own unique methodologies for identifying these losses. Consequently, the rates reported between studies vary tremendously. This situation has led to the apparent discrepancy often noted between doctors in their discussions on the rates of miscarriage.
The most widely accepted rate of loss for a single spontaneous abortion in an unselected population of couples (that is, regardless of characteristics associated with pregnancy loss) is about 15-20% (1 in 6) of clinically detected pregnancies (where the woman missed a menses or otherwise knew that she was pregnant).
Many pregnancies are lost prior to clinical detection but the incidence of these very early losses is not clear. A number of studies checked for pregnancy each month with a highly sensitive immunoassay from blood drawn or urine collected in sexually active women not using contraception. This research consistently demonstrates a high rate of unrecognized pregnancy in woman who are just a little late for the menstrual flow. Some studies report a total pregnancy loss rate (non clinical plus clinical) of more than 50% (1 in 2).
The chance of having a second spontaneous abortion with a history of only one isolated spontaneous abortion is generally considered to remain at 15-20% (for clinically recognized pregnancy). The incidence does not decrease (as if you used up your 1 in 6 and now must have 5 normal pregnancies) or increase significantly.
If there have been 2 spontaneous abortions in a row, then the most reliable information suggests that there is about a 35% chance (1 in 3) that the next pregnancy will be lost. Therefore, the loss rate is approximately doubled. If there have been 3 spontaneous abortions in a row, then it appears that the couple has a roughly 45-50% chance of a loss with the next pregnancy. There are reports indicating improvement in future pregnancy success for couples with recurrent pregnancy loss after there has been at least one prior live born for the couple (that is, a 40-45% loss rate if no live borns and only a 30% loss rate with a history of a prior live born). Therefore, the couple's prior reproductive history is also important.
The spontaneous abortion rate rises as the woman's age increases, with a gradual increase starting about age 30, more rapid increases after age 35, and much more rapid increases after age 40. The age related increases in spontaneous abortion rates appear to be predominantly due to chromosomal accidents around the time of fertilization, where the egg is given one too many or one too few chromosomes so that the resulting fertilized egg (embryo) has a lethal genetic abnormality. When women over 40 or 45 years age are recipients of donor eggs from younger women they do not have this increased spontaneous abortion rate. This suggests that the cause of this increase in loss rate is related to egg rather than uterine factors.
About 80% (4 in 5) of spontaneous abortions occur in the first trimester of pregnancy (in the initial 13 weeks gestation). In couples without a history of recurrent losses if a fetal heart beat (FH) is seen by ultrasonography at 6 weeks gestational age then there is a reduced loss rate to about 5% (1 in 20). There is a further reduction if an FH is seen at 8 weeks gestation to about 3% (1 in 33). Unfortunately, in couples with recurrent losses the loss rate is still about 4-5 times greater (about 20% or 1 in 5) even after seeing an FH. Of course, seeing the fetal heart beating is reassuring but not as encouraging as if seen in an unselected population.
The high level of uncertainty involving any pregnancy seems to warrant that couples remain cautiously optimistic when they recognize a pregnancy. Many couples do not announce that they are expecting until seeing the FH or the completion of the first trimester.