Estimated Date of Delivery

Issues of Concern

Clinical History and Physical Exam

An average pregnancy lasts 280 days from the first day of the last menstrual period (LMP) or 266 days after conception.[1] Historically, an accurate LMP is the best estimator for determining the due date. Naegele’s rule, derived from a German obstetrician, subtracts 3 months and adds 7 days to calculate the estimated due date (EDD). The obstetrician should get a detailed menstrual history, including duration, flow, previous menstrual periods, and hormonal contraceptives. These factors are used to determine the length of her cycles and ovulation period. There are several fallacies with Naegele’s rule. First, a woman may not accurately recall the first day of her menstrual cycle. Second, this method assumes a woman’s cycle is exactly 28 days, with ovulation occurring at day 14, however, it does not consider menstrual cycles with shorter or longer durations. Third, there are small variations in the duration between fertilization and blastocyst implantation. Last, this method cannot differentiate between menstrual bleeding and early pregnancy bleeding. Parikh’s formula was created to account for differences in menstrual cycle length. Parikh’s formula calculates EDD by adding nine months to the start of the last menstrual period, subtracting 21 days, then adding the duration of the previous cycle.

Other clinical methods that can calculate EDD include uterine size by pelvic examination using the over-the-curve technique. The over-the-curve technique involves using a measuring tape in centimeters from the superior aspect of the pubic symphysis to the top of the uterine fundus. Highly trained practitioners can palpate the uterine fundus as early as 12 weeks, commonly at the symphysis. Clinical landmarks approximate a 20-week gestation at the umbilicus, with a 16-week gestation at the halfway mark between the symphysis and the umbilicus. The over-the-curve technique is most practical from 16 to 38 weeks with a +/- 3 cm margin of error. Even though this method is informative, it is subject to error secondary to anatomical and/or structural changes in the uterus. For instance, if a patient has uterine fibroids, this can decrease the accuracy of this technique. For patients who have had a previous cesarean, the uterus is generally in a higher position because of pelvic adhesions. Other factors that decrease accuracy include body habitus, multiple gestations, and a retroverted uterus. Fetal heart tones are auscultated with the fetoscope at 19-20 weeks. Electronic Doppler devices can detect fetal heart tones as early as 8 to 10 weeks gestation. Understanding all the clinical estimators of calculating the due date have a margin of error of +/- 3 weeks is important.

Imaging

If a patient cannot recall the first day of her last menstrual period, has irregular cycles, or Naegele’s rule cannot be used, the next step is ultrasonography. Ultrasonography is best within the first half of the pregnancy. A Transvaginal (TV) sonogram can identify an intrauterine pregnancy (IUP) approximately 4 weeks after the LMP. An embryo is usually seen later between 5 to 6 weeks. Mean sac diameter (MSD) can be used early in gestation to estimate the due date if no embryo is on the sonogram. To calculate the MSD, the diameter sac measurement must be in the perpendicular plane, with a mean of three measurements. The calculation of EDD is from MSD by adding 30 mm. Given the large margin of error, MSD increases by 1mm per day during early pregnancy. MSD should not be used for final due date estimation. Additionally, anembryonic pregnancies can have a measurable MSD.

One should not use MSD if you can calculate the crown-rump length (CRL). CRL dating is the best method from 8 6/7 to 13 6/7 weeks. CRL is the longest straight-line measurement of the embryo from the outer margin of the cephalic pole to the rump. An accurate CRL includes the mid-sagittal plane, the genital tubercle, and the fetal spine in a straight line with a margin of error +/- 5 to 7 days.[1] An average of three measurements estimates the gestational age and EDD.  For second-trimester sonography, between 14 0/7 weeks and on, CRL is no longer the most accurate form of measurement. Instead, fetal biometrics, including biparietal diameter (BPD), head circumference (HC), abdominal circumference (AC), and femur length (FL), estimate gestational age and EDD.[2][3]

The BPD measurement criteria include an oval head shape instead of a round head shape; midline structures should be centered, measured at the level of the thalamus and cavum septum pellucidum, and should not include top orbits. The BPD measurement is from the proximal skull's outer edge to the distal skull's inner edge. The HC measurement criteria should include all those required for the BPD and be measured around the outer perimeter of the skull. The AC measurement criteria include the abdomen being round, not oval-shaped or squashed, true transverse image, images of ribs being symmetrical on both sides, measured at the umbilical level vein joins the portal sinus, calipers should go all the way to the skin surface. The FL measurement criteria include; perpendicular to the direction of insonate, ends should be sharply visible, and measurement should exclude epiphyses.

An official EDD is established after calculating the first-trimester sonogram EDD date and then using the LMP. If the LMP and first trimester EDD are within 7 days of each other, the LMP estimates the due date. The margin of error is reduced depending on when (ie, how early) the sonogram occurred. If the sonogram was before 8 6/7 weeks and the LMP is within 5 days, then the LMP estimates the due date; otherwise, use the sonographic EDD. If a sonogram took place after 14 0/7 weeks and if the EDD is within 14 days of the LMP, then the LMP estimates the due date. If the sonogram did not happen until the third trimester, then EDD can be calculated by LMP if the LMP is within 21 days. The American College of Obstetrics and Gynecology (ACOG) deems dating sonograms dated after 22 weeks suboptimal for gestational age (GA) measurement; this is because before 22 weeks GA, fetal biometry parameters are accurate within +/- 7 days to the CRL in the first trimester. After this range, variation in fetal size growth makes this less precise. If multiple sonograms exist, the EDD from the earliest sonogram detecting GA becomes the EDD.

Multi-fetal Gestations

As discussed above, the same rules apply to twins or higher-order gestations. The larger twin calculates the EDD if there is size discordance between the twins.

Assisted or artificial conception

The due date is determined by dates of ovulation, egg retrieval, insemination, cleavage stage, or blastocyst transfer. Assisted reproductive technology (ART) uses the principle of Naegle's rule as the foundation for estimating the due date. If fresh in vitro fertilization (IVF) is done, EDD is calculated by adding 266 days to egg retrieval/fertilization. If using a frozen embryo (day 3), EDD is 263 days from the date of embryo transfer to account for three days of embryo culture. If a day 5 blastocyst is implanted, add 261 days to this date to calculate EDD.

Clinical Significance

The determination of gestational age is vital to providing quality prenatal care. 

• Knowing the gestational age allows appropriate care for the mother without compromising maternal or fetal status.
• Knowing the gestation age allows for the correct timing of management, such as administering steroids for fetal lung maturity, starting ASA therapy with a history of pre-eclampsia in previous pregnancies, and starting Makena for previous preterm deliveries. 

Nursing, Allied Health, and Interprofessional Team Interventions

Correct EDD is paramount in coordinating care between healthcare professionals, including nurses, laboratory technologists, pharmacists, and physicians in different specialties. This will allow timely laboratory testing since most pregnancy tests are time-sensitive. Having accurate birth dating might decrease maternal/fetal morbidity and or mortality through timely consulting with experts in the field of maternal-fetal medicine, obstetrics/gynecology, oncology, or genetics. For example,  fetal genetic abnormalities can be detected in a timely fashion, providing the mother with sufficient time to make a lifetime decision.