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Questions to Ask About Changing Your Due Date

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Updated May 09, 2013

Written or reviewed by a board-certified physician. See About.com's Medical Review Board.

Monthly Calendar Circled Date

A due date is simply an educated guess as to the day that marks the middle of a time frame when you can expect your baby.

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Your due date is one of the first things calculated in your prenatal care. The due date is something that will help guide both you and your practitioner for gestational age-appropriate prenatal care, testing and ultimately your baby's birth. It is, therefore, very important to ensure that you have the most accurate due date possible.

Sometimes things happen, and you might find yourself being told that your due date is really a different date than you originally expected. In fact, as many as 26% of mothers have their due dates changed in pregnancy. Here is what you need to ask before agreeing to a change in date:

  • What is causing you to consider this change in my due date?
    Is there some new clinical data? Is my fundus (top of the uterus) measuring differently than expected? If so, what else might be causing that measurement to be off? One example might be a larger amount of amniotic fluid or even something as simple as your baby’s position.

  • How sure are you of this change?
    One common scenario that I hear is that a mom has been told that her due date is a certain date her whole pregnancy, but then at an ultrasound near the middle of pregnancy she is told a different date, usually only a matter of days different. But the truth is this change, if it is less than 14 days in either direction, is probably within the normal variation, because ultrasound at this juncture is limited in its use for dating a pregnancy. While being due a few days earlier might seem like a sweet deal now, it may be something that worries you if your pregnancy extends towards 42 weeks. This type of change might add extra risk to your pregnancy by facing an induction of labor that is unnecessary. (It is important to add that ultrasound in the first trimester is considered to be much more accurate for dating purposes, being within 10 days plus or minus the birth of the baby, but it is not recommended that all mothers have a dating screen because of such a slight difference in dates. This is used for women who have no data or very scanty data from irregular cycles.)

  • Are there other things that we may need to take into consideration before making this change? Could something else be causing us to consider this change?
    Most practitioners will look at a variety of factors as your pregnancy progresses to ensure that your due date is still accurate. Some of the things that are calculated into this clinical decision are:

    Other examples of things that might alter these readings would be the discovery of a twin pregnancy, uterine anomaly, maternal weight, etc.

  • Does this change alter my care? If so, how?
    Most of the time an altered due date will not immediately effect your prenatal care. The alteration potentially comes at the end of pregnancy when looking at intervening to end a pregnancy earlier 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 up front. When we look at a due date that is derived from your last menstrual period, we consider this to be plus or minus 14 days, meaning that we expect that your baby will show up between weeks 38 and 42 from the date calculated.

The average length of gestation is 266 days from conception, often reported as 280 from the last menstrual period. One issue with this measurement is that it assumes that you ovulate on day 14. This may not always be true. If you do not know when you ovulated, you can also consider using the length of your cycle and counting backwards 14 days.

Naegle’s rule is used to calculate a due date. This says that you take the first day of your period and count backward three months, and then add a week. So if my period started on February 1st, I would count back 3 months to November 1st and add 7 days for a due date of November 8th.

So for someone with a 32 day cycle, a good guess for date of ovulation would be day 18, which would alter your due date by 4 days. While four days may not seem like a lot of time in the beginning of your pregnancy, it may mean a lot of time toward the end of pregnancy when looking at whether to do an induction of labor or wait a few more days.

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.

Sources:

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.

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