Accurate determination of the estimated due date (EDD) during pregnancy has personal and social significance for the woman and her family, guides obstetric evaluation and intervention, and has research and public health consequences. The following is a summary of a recent, joint guideline on the method for estimating due date issued by the American College of Obstetricians and Gynecologists (ACOG), the American Institute of Ultrasound Medicine (AIUM), and the Society for Maternal-Fetal Medicine (SMFM) (2014).
If the last menstrual period (LMP) is known, calculation of the EDD based on LMP is the first step in determining the EDD. This predicts the EDD as 280 days from LMP, and is typically calculated using Naegele's Rule. Clinical dating based on Naegele's Rule has been used since the 1800s and is achieved by adding 7 days to the LMP and subtracting 3 months (Baskett & Nagele, 2000). As this calculation assumes accurate maternal recall of LMP; a regular, 28-day menstrual cycle; and ovulation on day 14, its accuracy is subject to individual menstrual cycle and follicular phase variations as well as reliable maternal recall of LMP (ACOG, AIUM, & SMFM, 2014).
Once the EDD is calculated based on LMP, biometric ultrasound measurements according to ultrasound timing by gestational age are used to confirm or adjust the EDD. Ultrasound measurement of the crown-rump length (CRL) in the first trimester is considered the most accurate estimator of the EDD. However, since the accuracy of CRL decreases past 84 mm (which corresponds to around 14 0/7 weeks), second- and third-trimester ultrasound estimates should be based on a composite of other biometric measurements (ACOG, AIUM, & SMFM, 2014). These include the biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL).
The LMP-determined EDD should be revised and redated to the ultrasound-determined EDD in the following circumstances (ACOG, AIUM, & SMFM, 2014):
* First-trimester ultrasound using CRL:
* Ultrasound dating performed before 9 0/7 weeks differs from LMP dating by more than 5 days.
* Ultrasound dating performed between 9 0/7 and 13 6/7 weeks differs from LMP dating by more than 7 days.
* Second- and third-trimester ultrasound using a composite of BPD, HC, AC, and FL:
* Ultrasound dating performed between 14 0/7 and 15 6/7 weeks differs from LMP dating by more than 7 days.
* Ultrasound dating performed between 16 0/7 and 21 6/7 weeks differs from LMP dating by more than 10 days.
* Ultrasound dating performed between 22 0/7 and 27 6/7 weeks differs from LMP dating by more than 14 days.
* Ultrasound dating performed at >=28 weeks differs from LMP dating by more than 21 days.
If the LMP is unknown, dating should be based on the earliest possible ultrasound, ideally in the first trimester using CRL (ACOG, AIUM, & SMFM, 2014). For pregnancies resulting from assisted reproductive technology (ART), the gestational age determined by ART should be used for the EDD. Third-trimester ultrasound assessment of gestational age is considered the least reliable, with an accuracy of +/- 21 to 30 days. Once the EDD is determined it should be documented in the medical record and discussed with the patient. Revisions to the EDD based on subsequent second- or third-trimester ultrasounds are discouraged (ACOG, AIUM, & SMFM).
Although determination of the EDD is a responsibility of the obstetric provider, nurses should be aware of the process of standards-based determination of EDD to give the most accurate and current information to women and their families and to enhance the nurse's advocacy role surrounding gestational age-based assessments and interventions. The full guideline (ACOG, AIUM, & SMFM, 2014) is available online at http://www.acog.org/Resources-And-Publications/Committee-Opinions/Committee-on-O.
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