Are Due Dates Accurate?

Ashley Strengholt
Sep 19
4 min read

Pregnancy due dates are something many expectant parents look forward to - marking the calendar, planning around the 40-week milestone, and imagining the big day. But how accurate are due dates really? The truth is that while they are useful for medical care, due dates are estimates, not guarantees.

How Due Dates Are Calculated

Traditionally, due dates are estimated using the last menstrual period (LMP), applying Naegele’s rule (add seven days to the first day of the LMP, subtract three months, and add a year). This assumes a 28-day cycle with ovulation on day 14 (Mittendorf et al., 1990).

Another common method is ultrasound dating, particularly in the first trimester. Research shows early ultrasound measurements (before 14 weeks) are more accurate for estimating gestational age than LMP, as fetal size becomes more variable later in pregnancy (American College of Obstetricians and Gynecologists [ACOG], 2017).

For pregnancies conceived with assisted reproductive technologies (ART), the date of conception is often known, allowing for more precise estimates (ACOG, 2017).

Why Due Dates Are Inaccurate

Even with advanced methods, due dates rarely predict the exact day of birth. Several factors contribute to this inaccuracy:

Cycle variability: Not all women have a 28-day cycle or ovulate on day 14. This makes LMP-based dating less reliable, especially for people with irregular cycles (Jukic et al., 2013).
Variation in implantation and growth: After fertilization, embryos implant at slightly different times. Later in pregnancy, fetuses also grow at different rates, which can affect ultrasound dating (Jukic et al., 2013).
Recall error: Many women do not remember the exact date of their LMP. A study in South Africa found that LMP-based dating misclassified more preterm and post-term births compared to ultrasound dating (Majola et al., 2021).
Natural differences in pregnancy length: Even when conception is precisely known, pregnancies vary widely. Jukic et al. (2013) found that the median length of human pregnancy from ovulation to birth was about 268 days (38 weeks + 2 days), but ranged by as much as five weeks among healthy women.

How Often Are Due Dates Correct?

Studies consistently show that only a small percentage of babies are born on their exact due date. Research indicates that only about 5% of births occur on the estimated date (Mongelli & Gardosi, 1996). Most babies are born within a two-week window on either side, but many arrive earlier or later - even when dating is precise.

Clinical Importance of Due Dates

Even though they are not exact, due dates play an important role in medical care. Timing of prenatal screening, assessing fetal growth, and making decisions about induction or interventions often depend on gestational age. Because of the risks of intervening too early or too late, ACOG (2017) recommends using first-trimester ultrasound to establish or confirm due dates whenever possible. If a pregnancy has not had an ultrasound before 22 weeks, it should be considered “suboptimally dated” (ACOG, 2017).

A Better Way to Think About Due Dates

Instead of viewing due dates as a deadline, it is more accurate to think of them as a range. Many organizations consider a pregnancy “term” anywhere from 37 weeks to 42 weeks, with “full term” specifically defined as 39 weeks to 40 weeks + 6 days (Spong, 2013).

Knowing this range may help reduce anxiety when a baby does not arrive “on time.” For many women, previous pregnancy lengths can also provide clues, since individual patterns may repeat (Oberg et al., 2017).

Conclusion

Due dates are valuable tools in pregnancy care, but they are not precise predictors of when labor will begin. Only a small percentage of babies are born exactly on their due date, and natural biological variation makes it impossible to pinpoint the day with certainty. By thinking of due dates as estimated ranges, families and providers can set more realistic expectations and reduce stress during the final weeks of pregnancy.

References

American College of Obstetricians and Gynecologists. (2017). Methods for estimating the due date. ACOG Committee Opinion No. 700. Obstetrics & Gynecology, 129(5), e150–e154.
Jukic, A. M., Baird, D. D., Weinberg, C. R., McConnaughey, D. R., & Wilcox, A. J. (2013). Length of human pregnancy and contributors to its natural variation. Human Reproduction, 28(10), 2848–2855.
Majola, T., Moodley, D., Esterhuizen, T., & Sartorius, B. (2021). Accuracy of gestational age estimation methods and implications for preterm birth classification: Findings from a prospective cohort in South Africa. BMC Pregnancy and Childbirth, 21(1), 511.
Mittendorf, R., Williams, M. A., Berkey, C. S., & Cotter, P. F. (1990). Length of human pregnancy and accuracy of last menstrual period. Obstetrics & Gynecology, 75(5), 766–770.
Mongelli, M., & Gardosi, J. (1996). Reduction of false positive diagnosis of preterm labour by ultrasound biometry. British Journal of Obstetrics and Gynaecology, 103(6), 528–533.
Oberg, A. S., Frisell, T., Svensson, A. C., & Iliadou, A. N. (2017). Maternal and fetal predictors of prolonged pregnancy. Epidemiology, 28(4), 547–554.
Spong, C. Y. (2013). Defining “term” pregnancy: Recommendations from the Defining “Term” Pregnancy Workgroup. JAMA, 309(23), 2445–2446.