Pregnancy Dating Criteria

Clinical description

Dating the pregnancy accurately is of critical importance since management of risks during pregnancy and labour often depend on the gestational age (GA) (i.e., when and if to give steroids, conservative management versus early delivery, identifying intrauterine growth restriction, and induction of labour).

Ultrasound measurement of the embryo or fetus in the first trimester (up to 13 6/7 wks) is the most accurate method to determine GA.

Fundal height is not an accurate measurement for gestational age and should not be relied upon independent from LMP and US measurement.

Signs and symptoms

HistoryAsk the date of the first day of the woman's last menstrual period (LMP).

• A careful history must be taken from the patient to determine the first day of her last period and that her periods are

regular (appear every month while not on contraception).

• Clarify whether the LMP was normal or not, such as a post-hormonal contraception period.
• Find out if an early pregnancy test was performed.

Exam If < 14 wks: The average of three separate crown-rump length (CRL) measurements in a midsagittal plane should be used to determine GA.

• If the GA determined by the US differs from the GA from the LMP by more than 7 days, the GA and EDD should

be changed to what is determined by the US.

• If the dating agrees within 7 days, the LMP dating should be used.
• If the US is done at less than 9 0/7 wks, and there is a discrepancy of more than 5 days from LMP dating, the dates

should be based on the US.

If ≥ 14 wks:Measure fetal biometry (head circumference, biparietal diameter, abdominal circumference, and femur length).

• If the GA determined by the US differs from that of the LMP by more than 14 days, the GA and EDD should be

changed to what is determined by the US.

• If the dating agrees within 14 days, the LMP dating should be used.

Amount of discrepancy from LMP that would suggest using the US dating:

• >16 wks: More than 10 days
• >22 wks: More than 14 days
• >28 wks More than 21 days

If the patient is unsure of her LMP, dating should be determined by the earliest possible US.

If an earlier US was done before your present evaluation, GA should not be changed.

The earlier the US, the more accurate the dating. Instead, consider whether there is a growth disturbance.

If the US you are using does not calculate GA for you, use this website: perinatology.com

Treatment

Document clearly what the correct gestational age and estimated date of delivery is in the health passport. Tell the patient if this is the first US for dating she has had and let her know that her EDD does not change from this.

• If only one measurement can be taken:
• Head circumference best predicts GA if GA 14-24 wks, but can have a margin of error of 2 weeks
• Femur length best predicts GA in third trimester, but can have a margin of error of 3-4 weeks