Noone really knows how often human pregnancies are lost. Investigation
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 previable pregnancy
losses to a centralized agency (as there is with live birth statistics).
Most of the research on incidence rates of pregnancy loss incorporate
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
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 (nonclinical 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
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.