Babies choose when they want to be born. My favorite description of this from a 1951 astrological treatise (Outrageous Fortune by Myra Kingsley) is as follows:
*In Maeterlinck's fairy tale, "The Blue Bird", there is a charming scene in the Azure Palace, where the babies who are yet to be born are waiting. Father Time is about to draw the bolts that open the Gates of Dawn and is assembling his charges destined for Earth, shining brightly in the infinity of space below. He sorts out the children-the engineers, the doctors, the shepherds of the future-who are to leave, some impatient, some reluctant. "It is from here that all children come who are born upon the Earth," Light remarks to the visitors. "Each awaits his day. When the fathers and mothers want children, the great doors are opened and the little ones go down." The doors open; the children go down; in the distance swells a song of gladness and expectation. This is the song of the mothers coming out to meet the children.*
Of interest in that book: The author looked at the chart of very young Prince Charles and predicted that he may never "come to the throne". Babies always have a connection with the parents' charts. However, it can be something as simple as the baby's Ascendant is equal to the parent's north node. Or the child's Sun is on the parent's midheaven, or on the Jupiter Uranus midpoint. The Mars Jupiter and Venus Uranus midpoints are often triggered at birth, but that still does not give one the tools to say in advance the day, much less the time, that the baby will arrive. This is really up to the baby who is awaiting the perfect moment. I do think it is possible to predict the year and month when a child will be born, using a variety of predictive techniques. But when it comes down to the day, even astrologers have to be patient and watch the moon signs change as they wait. If you are an astrologer reading this and have a fool proof system, let me know.