Bringing a human child to term generally takes place over a period of nine months. They are marked by different stages, from 1st to third, and are marked by different events and expectations for mother and child. While no two instances are the same, this process is generally similar enough to be predictable. Babies are generally in the womb for the same amount of time.
In an alternate world, women are regularly with child for more than a year, with the most extreme verified case being more than three. The "average pregnancy" lasts, about nine months, but comes with a "plus or minus six months" qualification. Because of gestational periods being more unpredictable, it becomes essentially impossible to determine when any given child will be born. Standard deviations are quite large, with any one individual having multiple several prrgnancies with various gestational times. This has no overall effect on the child itself.
What alterations to the biology of a species would lead to this development taking place?