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Pregnancy Due Date

Your pregnancy due date may be calculated in several ways, a pregnancy calendar, pregnancy calculator, or manually (based on ovulation or an average). Learn tips on how to calculate your pregnancy due date, or approximate date to expect your baby's birth.

The date referred to as a “pregnancy due date” is an estimate of when a baby might be born, given the available information. This article helps explain how the pregnancy due date is calculated and what it means.

Variation In Pregnancy Due Date Is to Be Expected

Ovulation is not a mechanical procedure that takes place like clockwork, the same for all women. No, ovulation can occur on any day of a woman’s menstrual cycle from the eleventh to the twenty-first, and an egg’s fertility can last up to 24 hours, extending the possibility to the twenty-second day.

Two Often-Used Ways to Calculate Pregnancy Due Date

There are two divergent methods of calculating a pregnancy due date from dates.

Using Only One Date to Determine Your Pregnancy Due Date

 In the first method, all the calculations are made based on the first day of the last menstrual period. This is the method used by some online pregnancy due date calculators that only ask you to enter the first day of your last period. In this method, it is assumed that ovulation and conception both occurred 14 days after the date given. So if you were to give January 1, 2010 as the date, the calculator would tell you that your baby was due approximately 40 weeks later (14 days till the date of conception and 38 weeks of pregnancy), your pregnancy due date would be on or about October 8, 2010.

• Using a Date and an Average to Determine Pregnancy Due Date

In the second method, the initial day of the last menstrual period is used, but that information is combined with the average length of a woman’s period. This can give a more accurate estimate, because it can help provide a better guess than “14” days after the first day of the cycle as the day of ovulation. This is because the phase of the ovulation cycle after ovulation, called the luteal phase, has a more compact timeframe that is believed to have limitations of 12 to 16 days.

Thus, in this method, if a woman’s periods averaged 33 days, the estimated day of ovulation (and therefore conception) would be arrived at by subtracting from 33 for an estimate of ovulation (and therefore conception) later in the month. One online calculator, given January 1, 2010 as the first date of the last period and 33 as the average number of days in the woman’s cycle, calculates a pregnancy due date of October 14, 2010, six days later, based on the added piece of information.

Using the Date of Ovulation to Determine Pregnancy Due Date

By using fertility awareness methods, also referred to as Natural Family Planning (NFP), the Ovulation Method, the Billings Method, or the Sympto-Thermal Method, woman may use a combination of calendar, basal body temperature, cervical mucus observation, and other information to come to a greater appreciation of the exact timing of ovulation, narrowing down the time period for conception. An ovulation predictor kit may also be of assistance.

Even knowing the exact moment of ovulation does not, however, usually give the date of conception with complete certainty, given that eggs can be fertilized for 24 hours and sperm can live for 3 to 5 days. But it may help you calculate a more precise pregnancy due date.

Thirty-Eight Weeks, Give or Take

Or, you might say that pinpointing conception would give a more precise date if we knew exactly what goes into the determination of the beginning of labor. The fact is, this is something that is not yet known. However, besides the “pregnancy due date” that is often calculated by the mother, even as she is confirming the pregnancy, and again at her prenatal visit, a couple of other elements can contribute to a more precise guesstimate:

  • Early in the pregnancy: A pregnancy ultrasound and other measures of the baby can help determine its gestational age. This estimate can be used to confirm the expected pregnancy due date.
  • Late in the pregnancy: Preliminary signs of labor, such as effacement, dilation, and loss of the mucous plug can all help to gain a more specific idea of how things are progressing as the initially calculated pregnancy due date approaches.


Related Article: Pregnancy Ultrasound >>