When Your Doctor Changes Your Pregnancy Due Date

Pregnancy Due Date Calendar

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Your due date is one of the first things that will be calculated in your prenatal care. The due date guides you and your provider in gestational age-appropriate prenatal care, testing, and ultimately your baby's birth. That's why it's important that your due date is as accurate as possible.

However, it's not always straightforward. You might find yourself being told that your due date is a different date than you originally expected. In fact, as many as 26% of people have their due dates changed in pregnancy. Here are a few questions to ask your provider before having your due date changed.

Causes of Changing a Due Date

Your provider will look at several factors as your pregnancy progresses to ensure that your due date continues to be accurate. Some of the factors that they will use to determine the accuracy of your due date include:

  • Fetal heart tones: When did a provider hear the baby’s heartbeat and which instrument did they use?
  • Fundal height: The fundus (top of the uterus) might measure differently than expected based on your predicted due date.
  • Quickening: When did you first feel the baby move?

Other factors, such as the discovery of a twin pregnancy, uterine anomaly, and your weight can also affect the calculation of your due date

Due Date Accuracy

One common scenario is that a person has been told that their due date is a certain date throughout their entire pregnancy, but then an ultrasound near the middle of the pregnancy, they are told a different date—usually, it's only a matter of days different.

This change, if it is less than 14 days in either direction, is probably within the normal variation. Ultrasounds at this point are limited for dating a pregnancy. The specificity of ultrasound dating a pregnancy varies with each trimester.

The most accurate dating is in the first trimester (which varies plus or minus seven days) and the least accurate dating is in the third trimester.

Your provider will recommend that you have an ultrasound during the first trimester of pregnancy to see if the due date calculated from the day of your last menstrual period needs to be adjusted.

Impact on Prenatal Care

An altered due date usually won't have an immediate effect on your prenatal care. The alteration potentially comes at the end of pregnancy when you might be looking to move toward delivery because of the new due date. If you do decide to go with an amended due date, this is something to remember if it comes time to start talking about induction of labor.

One of the best ways to avoid changes in due dates is to ensure that you have the most accurate data. When your provider looks at your due date derived from your last menstrual period, it's considered to be plus or minus 14 days. This means that your provider expects that your baby will show up between weeks 38 and 42 from the date calculated.

The average length of gestation is 266 days from the date of conception, which is reported as 280 from the last menstrual period. However, there is an issue with this measurement because it assumes that you ovulated on day 14, which might not always be true. If you do not know when you ovulated, you can consider using the length of your cycle and counting backward 14 days.

Naegle’s rule is used to calculate a due date. You take the first day of your period and count backward three months, and then add seven days.

For example, if your period started on February 1st, you would count back three months to November 1st and add seven days for a due date of November 8th.

For someone with a 32-day cycle, a good guess for the date of ovulation would be day 18. This would alter your due date by four days. While four days might not seem like a lot of time at the beginning of your pregnancy, it could translate to a lot of time toward the end of pregnancy when your provider might be trying to decide whether to induce labor or wait a few more days.

A Word From Verywell

Never hesitate to ask questions about a due date change. The information can only help you understand what is going on and why this is being discussed. It can help you move forward with the healthiest pregnancy possible.

2 Sources
Verywell Family uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Declercq ER, Sakala C, Corry MP, Applebaum S, Herrlich A. Major Survey Findings of Listening to Mothers(SM) III: Pregnancy and Birth. J Perinat Educ. 2014;23(1):9-16. doi:10.1891/1058-1243.23.1.9

  2. Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ. Length of human pregnancy and contributors to its natural variation. Hum Reprod. 2013;28(10):2848-2855. doi:10.1093/humrep/det297

Additional Reading
  • Ananth, Cande V. (2007). Menstrual versus clinical estimate of gestational age dating in the United States: temporal trends and variability in indices of perinatal outcomes. Paediatric and Perinatal Epidemiology, 21, 22-30. doi: 10.1111/j.1365-3016.2007.00858.x
  • Declercq ER, Sakala C, Corry MP, Applebaum S, Herrlich A. Listening to Mothers III: Pregnancy and Childbirth. New York: Childbirth Connection, May 2013.
  • Gabbe SG , Niebyl JR, Simpson JL, Galan H, Goetzl L, Jauniaux ER, Landon M. (2007). Obstetrics: Normal and Problem Pregnancies (5th ed.): Churchill Livingstone.
  • Goldenberg, Robert L., McClure, Elizabeth M., Bhattacharya, Anand, Groat, Tina D., & Stahl, Pamela J. (2009). Women's Perceptions Regarding the Safety of Births at Various Gestational Ages. Obstetrics & Gynecology, 114(6), 1254-1258.

By Robin Elise Weiss, PhD, MPH
Robin Elise Weiss, PhD, MPH is a professor, author, childbirth and postpartum educator, certified doula, and lactation counselor.