The longer we leave it to date a pregnancy by ultrasound, the less accurate the dating will be. Therefore, the earlier the scan, the more accurate the due date as there is much less variation in the size of the baby. If we think about the size of a term baby, normal can be anywhere between 2.5kg and 4.5kg, but we know that all babies start as one fertilised cell. Why do due dates vary so much? This is a good question. Babies are generally born between 37 and 42 weeks, however some arrive earlier (pre-term: less than 37 weeks) and some are later (post-term: more than 42 weeks).In terms of pregnancy care, it is important to have a due date as this date represents the 40-week mark, and provides an accurate reference point from which to determine the timing of tests and scans and to measure appropriate growth and development of the baby. It can be helpful to think of the due date as the point at which a woman is 40 weeks pregnant, because as you rightly say, babies come when they’re ready, so the ‘due date’ is not really the date at which a baby will be born. Subsequent ultrasounds do not change the due date. It does sound confusing, doesn't it? If we use an ultrasound for dating a pregnancy, we only ever use the very first pregnancy ultrasound for dating purposes.
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