Therefore, an increase may not be much to get all worked up about. Even an increase double or triple the rate of the 25 year old population still means the absolute risk is low. Not something to loose sleep over. Secondly, the reason more older women have complications during pregnancy is not all due to age. It is closely related to the increased rates of complications and diseases and problems as we age.
Older women are simply more likely to have a medical problem to begin with.
Pregnancy is a major strain on the heart and blood vessels, and in turn, on all the organs in the body. Nearly any pre-existing condition will be worsened during pregnancy and make a more difficult pregnancy. Like many age-related correlations in medicine, the risk depends on things that happen to us over time, not just age itself. Older mothers are more likely to have had other children. Some of the complications, like the placement of the placenta over the cervix, are more likely in women who have had many prior pregnancies.