There was an article, or chapter in a book - something I read years ago - that addressed the duration of gestation (meaning - how long it takes for an infant to develop in the womb before birth). The author stated that gestational time is not fixed - that the birthdate of a baby depends upon the genes in the family, the circumstances (such as nutrition and health) during the pregnancy, and other conditions. He brought up deer as an interesting example. Deer, compared to many other animals, are noted for variation of time between fertilization of the egg, and giving birth. If the weather is good, and food and water are in steady, reliable supply, gestation is short. During a dry year when food and water are scarce - the time before birth can be extended as long as two months, until food and water are more readily available.

I also read, regarding human beings, that even when the fertilization date is exact, the timing of the birth can vary considerably from one mother to the next. One cannot reliably predict date of birth, it's always an ETA (estimated time of arrival). This can be hard on people who like to know in advance, to do everything on schedule as carefully planned! But there is a wisdom in how nature works.