Many couples blame themselves (often harshly) for their pregnancy
losses. In fact, it is rare that either member of the couple has
done anything that would result in a pregnancy loss. Additionally,
the actual incidence of pregnancy loss in the United States is
much higher than typically thought. The result is that many couples
might benefit from knowledge of the recognized causes for pregnancy
There is a major difference in both "incidence rates"
and "causes" for single spontaneous abortions and recurrent
spontaneous abortions. Recurrent abortion is typically defined
as three or more consecutive (in a row) pregnancy losses that
occur prior to fetal viability (usually 20 weeks gestation or
a fetal weight of 500 grams). The reason for this criteria is
the reports of a significantly higher chance for further pregnancy
loss following the loss of "three in a row."
The two major clinically important categories of causes for spontaneous
abortion (miscarriage) are fetal and maternal.
Fetal causes include the genetic composition of
- human live borns have a very low percentage of chromosomal
abnormalities (about 0.6% or 1 in 170). This low percentage indicates
that almost all chromosomal abnormalities are lethal and aborted
early in pregnancy.
- The only chromosomal abnormalities (other than those involving
the X and Y sex chromosomes) that might result in a human live
born are trisomy 21 (three of the 21 chromosome, known as "Down's
syndrome"), trisomy 18 (three of the 18 chromosome, known
as "Edward's syndrome" and all die during infancy) and
trisomy 13 (three of the 13 chromosome, known as "Patau syndrome"
and all die during infancy).
Maternal causes include abnormalities in the "environment"
in which the embryo and fetus develops. Known maternal causes
related to an action of the mother are uncommon, but can include
- heavy smoking (uncommon for this to result in a loss)
- alcohol abuse (uncommon for this to result in a loss)
- irradiation or exposure to chemical toxins
- medications known to be teratogenic (cause fetal malformation)
Other maternal causes which are not related to any conscious activity
of the mother or couple include
- anatomic abnormalities (typically uterine)
- hormonal imbalances (typically in progesterone)
- immunologic system errors (autoimmune and alloimmune)
- serious or life threatening maternal disease
By far the most common causes for spontaneous pregnancy loss are
fetal not maternal. It is difficult for a woman with an undesired
pregnancy to consciously create an unfavorable environment for
the pregnancy to successfully force a miscarriage.
Often couples blame themselves for "doing something"
that must have resulted in the pregnancy loss. Focusing on themselves
(often harshly) for doing something wrong is unfortunate since
- it adds guilt on top of an existing emotionally charged situation,
which is counterproductive and may delay or arrest recovery from
- it is misdirected since very few losses are related to conscious
- it often assumes that such losses are rare events when in
fact they are common (but not commonly discussed)