How accurate are dating scans in pregnancy

The dating scan is an ultrasound scan that can be done as early as six weeks after the last period, but the dating scan will be most accurate when done between 8 and 12 weeks gestation.

At this time, the fetus is a little larger and easier to measure.

In a normal pregnancy it does not matter what the dates are, but if anything should go wrong it becomes very important as to what treatments may be prescribed.

Traditionally the last menstrual period has been used to date a pregnancy, but this may be highly inaccurate.

Ultrasounds done later in the pregnancy are less accurate for dating, so if your due date is set in the first trimester, it shouldn't be changed.

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.

The software has certain measurement scales based on data from large populations, and your baby’s measurements are put into this scale.

In other words, by comparing your baby’s measurements to the data from this large collection of measurements, the ultrasound can then tell how far along your baby is.

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