Accuracy of pregnancy dating

by  |  28-Nov-2017 12:29

For most mums-to-be, the first scan will be a dating scan at between 10 weeks and 13 weeks plus six days of pregnancy.

However, the timing of your first scan will also depend on how your pregnancy is going, and where you live.

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You can calculate your due date by subtracting three months from the first day of your last menstrual period (LMP) and then add a week. A pregnancy is based on being 280 days long, which is 40 weeks (more like 10 months not 9! When we give you a due date we consider 37 to 42 weeks to be full-term, so even if your baby is born two weeks before your due date it is not considered premature.

The accuracy of using your LMP to date your pregnancy depends on having regular cycles and assumes you ovulated (released an egg) on day 14 of your cycle and conceived right away - ­ none of which may be true.

Not all units offer an early scan if all is going well in this pregnancy, though.

Ask your GP if you want the reassurance of an early scan. The scan's main purpose is to work out accurately how many weeks pregnant you are and estimate your due date.

You probably know when your LMP was, but not when you actually conceived.

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