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Gestational Age

Gestational age is important to determine overdue status, for planning the birth and identifying the proper windows for screening.

An estimated date of delivery, or gestational age, can be determined by Nagele's Rule: from the date of the LMP by adding 7 days and subtracting 3 months from women with 28-day cycles.

Pregnancy wheel

Cycle regularity

ask about regularity
no contraception, regular

if they do not know, the most accurate method is the earliest done ultrasound

conception date: add two weeks

There are various ways of determining gestational age:

LMP
fundal height
fetal heart beats (at 10 weeks via Doppler)
quickening (at 17-18 weeks)
ultrasound (fetal crown-rump length at 5-12 weeks; biparietal diameter at 13 weeks until delivery, though less reliable as time goes on)

Nagele's Rule: add 9 months and 7 days to the LMP.

Ultrasound determination of dates

first trimester +/- 3-5 days: uses crown-rump length
second trimester +/- 7 days
third trimester: +/- 14 days

Routine Ultrasound

18-20 weeks
offered to everyone