## How is gestational age measured?

The gestational age of the pregnancy (or how far along the pregnancy is) can be determined accurately early in pregnancy by ultrasound measurements of the gestational sac diameter or the length of the fetal pole (CRL). Charts have also been developed for this purpose, and some simple rules of thumb can also be effectively used.

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## Gestational sac

There is a simple mathematic formula to determine the gestational pregnancy age from measurements of the gestational sac. You take the mm measurements of the gestational sac (GS) and add it to 28 days (or 4 weeks). That will provide you with a general estimate of how far along the pregnancy is: Gestational age = 4 weeks (28 days) plus (mean sac diameter in mm x days). This relies on the growth of the normal gestational sac of 1 mm per day after the 4th week of gestation. For example, a gestational sac measuring 11 mm would be approximately 5 weeks and 4 days gestational age. (4 weeks plus 11 days = 5 weeks and 4 days).

Another example: The GS gestational sac measures 4 mm then the total days are 32 days (28 + 4) or 4 weeks and 4 days.

 Gestational sac (mm) Days Weeks 1 mm 29 days 4w 1d 2 mm 30 days 4w 2d 3 mm 21 days 4w 3d 4 mm 32 days 4w4d 5 mm 33 days 4w 5d 6 mm 34 days 4w 6d 7 mm 35 days 5w 0d 8 mm 36 days 5w 1d 9 mm 37 days 5w 2d 10 mm 38 days 5w 3d 11 mm 39 days 5w 4d 12 mm 40 days 5w 5d 13 mm 41 days 5w 6d 14 mm 42 days 6w 0d 15 mm 43 days 6w 1d

## Crown-rump length

Gestational age = 6 weeks plus (CRL x days). This relies on the growth of the normal fetus of 1 mm per day after the 6th week of gestation. For example, a CRL of 16 mm would correspond to a gestational age of 8 weeks and two days (6 weeks plus 16 days = 8 weeks and 2 days).

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