30. Obstetric estimate of gestation at delivery

The best obstetric estimate of the infant’s gestational age (OE) in completed weeks is based on the clinician’s final estimate of gestation.

For the clinician:

The best estimated delivery date (EDD) is determined by the date of the last menstrual period (LMP) if confirmed by early ultrasound. If ultrasound is not performed or is unknown, the best EDD is determined by the LMP. If the LMP is unknown or inconsistent with the early ultrasound, the best EDD is determined by early ultrasound. For deliveries resulting from the use of assisted reproductive technology, the best EDD is based on the known date of fertilization.*

Accurate pregnancy dating is important to improve outcomes and is a research and public health imperative. The best estimated due date should be documented clearly in the medical records.*

Enter the final best obstetric estimate of the infant’s gestational age in completed weeks.

The OE should be based on the clinician’s estimate of gestational age at delivery. Look for the most recent gestational age estimate in the records (this can often be found in the labor and delivery records under “gestational age”).

If the most recent gestational age is dated on or after the infant’s date of delivery, enter this estimate.

If the most recent gestational age is dated before the date of delivery, add the number of days between the most recent gestational age and the date of delivery to the gestational age estimate. For example, if the most recent gestational age in the records is 32 weeks, 5 days and is dated 3/24, and the date of delivery is 3/31, add 7 days to the gestational age for the final total of 33 weeks, 5 days.

The gestational age for the OE is reported in completed weeks. If the OE includes a fraction of a week (e.g., 33 weeks, 5 days) always round down to the nearest whole week (e.g., 33 weeks).

If a final best obstetric estimate of gestation cannot be found, search for the best obstetrical estimated due date (EDD) and calculate the OE as shown below.

If both the EDD and the OE are not known, enter “unknown.”

If the date of delivery occurs before the EDD, then use the following calculation:

1. Find the most recent best  EDD (this may only be found in the prenatal care records) and the date of delivery.
2. Calculate the difference between the EDD and the date of delivery in days (e.g., EDD of 1/7/2015 – date of delivery of 12/30/2014 = 8 days)
3. Subtract difference between the EDD and the date of delivery from 280 days (e.g., 280 days – 8 days = 272 days)
4. Divide the total number of days from step 3 by 7 days (e.g., 272 days ÷ 7 days = 38.9 weeks) to determine the OE in weeks.
5. Enter the OE in completed weeks (e.g., 38 weeks).

If the date of delivery occurs after the EDD, then use the following calculation:

1. Find the most recent best  EDD (this may only be found in the prenatal care records) and the date of delivery.
2. Calculate the difference between the date of delivery and EDD in days (e.g., date of delivery of 1/5/2015 – EDD of 12/28/2014 = 8 days)
3. Add the difference between the date of delivery and the EDD to 280 days (e.g., 280 + 8 = 288 days)
4. Divide the total number of days from step 3 by 7 days (e.g., 288 days ÷ 7 days = 41.1 weeks) to determine the OE in weeks.
5. Enter the OE in completed weeks (e.g., 41 weeks).

1st  OB admission H&P under –

• Weeks
• Gestational age

Gestation _____ weeks (wks)

_____ weeks gestational age

GA – Gestational age

EGA – Estimated gestational age