New studies show that miscarriages occur in the animal kingdom as well as in humans. In the animal kingdom at least, there may some good evolutionary reasons at work.
In 1959, Dr. Hilda Bruce discovered the Bruce Effect. This is the tendency of pregnant female lab mice to miscarry when new males are introduced to the group. When female mice are exposed to certain pheromones in male mouse urine they may experience a drop in the hormone progesterone which is necessary to maintain pregnancy in mice (and people).
Although scientists seem to agree the this may serve an evolutionary purpose in the mouse population, they seem to be divided about whether it is more beneficial for male or female mice. It is most beneficial for dominant males to produce offspring. If the female mouse is pregnant with another male's offspring, it benefits the new male to end that pregnancy and replace it with his own.
Male mice (indeed males of many species) often kill babies to which they have no biological connection. Therefore it may benefit the female mouse to not complete a pregnancy where the babies are likely to be killed.
Although scientists have been aware of the Bruce Effect for many years, until recently, it was thought to exist mainly in mice and primarily in those in captivity. However, recent studies suggest that it is also present in Gelada monkeys. Researchers are unsure exactly how the Bruce Effect is triggered in the monkeys. Although it seems to be a biochemical response, it may prove to be somewhat different than it is in mice.
In the Gelada monkey study, pregnant females quickly miscarried when a new dominant male was introduced. As with mice, dominant male Gelada monkeys often kill offspring which are not theirs. The study showed that female monkeys who experienced this loss of their babies took longer to get pregnant again than the females who miscarried.
Other studies suggest that the Bruce Effect may also occur in lions and horses.
Obviously miscarriage in humans is a much more complicated issue involving many more potential risk factors and an emotional component as well. Still, it is interesting to know that miscarriage occurs elsewhere in the animal kingdom.