Calculate Your Due Date

Once you find out you're pregnant, one of the first questions you may ask is when is your due date. Since the date of conception is rarely known, learning how to calculate your due date is based on if you can remember the date of your last menstrual period (LMP) before the pregnancy test read positive. However, due dates are just an estimation as to when the baby may arrive. Only about 5% of babies are actually born on their due date, but you need to have a general idea of when your baby should arrive to monitor its progress and development.

Steps

Calculating Based on Your Last Period

  1. Determine the date of your last menstrual period (LMP) before you found out you were pregnant. This method works best for women whose menstrual cycles last around 28 days.
  2. Add 40 weeks from that date to determine the due date. Pregnancy usually takes 9 months, 40 weeks, or 280 days — give or take a few weeks — from the LMP.
  3. Alternately, remember Naegele's Rule. An easier way in figuring out due dates is to take the first day of your last period, count backward 3 months, add 7 days, and add an entire year. This is called Naegele's Rule. (Alternatively, to get the same result, you can take your LMP, add 1 week, and add 9 months.)
  4. Choose other methods to calculate your due date if you cannot remember the date of your LMP or have irregular periods. In this case, it may be difficult to determine the due date without help. To get a more accurate measure, you may want to wait until you see the doctor and have an ultrasound to determine the baby's gestational age and your due date.
    • You may, however be able to hit upon a rough estimate. Most women ovulate 14 days before their period begins. So if your cycles are 40 days, then you probably ovulated on the 26th day. If you know the day(s) of ovulation, just add 266 days to that date to determine a very rough estimate.

Using an Online Calculator

  1. Search for a free online due date calculator. Search in the term "free due date calculator" online and click on a reputable looking source.
  2. Provide date of conception or LMP. Since proving the date of conception is notoriously finicky, many online calculators suggest you stick with your LMP, which it then uses to calculate your due date.

Calculating at the Doctor's Office

  1. Visit a doctor. Estimating your due date on your own or using a website can give you a rough idea as to when you might deliver, but a doctor can confirm how far along you are. At your first prenatal appointment, your doctor may order a vaginal ultrasound to more accurately determine your due date.
    • Ultrasounds are more accurate in determining due dates when done early in the pregnancy. Later ultrasounds help in seeing if the fetus is growing well, but it doesn't help in determining fetal age.
  2. Be prepared for a little uncertainty. A due date is not set in stone, nor should it be: your baby will pop out whenever she is ready to, and hopefully not a day sooner or later. As such, a due date is always a work in progress. Some pregnant women even have their due dates changed mid-term.



Tips

  • A normal pregnancy can last anywhere from 38 to 42 weeks. The 40-week estimate is only the average length of a pregnancy.
  • Determining your due date on your own is ideal if you have a 28-day cycle. If your periods are irregular, it may be best to consult a doctor to determine how far along you are.
  • Your due date may change if you are expecting multiples, such as twins or triplets. Most multiple pregnancies do not make it to 40 weeks, and some doctors may want to induce you based upon the progress made by the fetuses.

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Sources and Citations

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