Due Date Calculator

Dominique W. Brooks
hospital birth

When you find out you are pregnant, you may be wondering how a due date calculator can help you determine when you will be giving birth. You may also want to know when to ask for time off work to give birth to your baby. Unfortunately, babies do not follow an exact calendar. While you may not want to book your hospital room just yet, you can calculate an estimated date of birth with a due date calculator.

LoveToKnow Pregnancy Due Date Calculator

Use the calculator below to estimate your due date.

Other Free Pregnancy Calculators Online

There are several free online pregnancy calculators that you can use. Here are some examples:

There are others available as well. Most of these calculators require specific information in order to determine your potential due date. Along with the results to your online due date calculation, the website may also offer you a pregnancy calendar overview, links to other pregnancy related information, and resources for receiving online support during your pregnancy.

Information Needed for a Due Date Calculation

Many online due date calculators only require the date your last period started and the number of days in your normal menstrual cycle. Other online due date calculators give you the option of calculating your due date based on the following:

  • First date of your last menstrual period
  • Likely date of conception
  • Estimated due date

You will still be asked for the normal number of days in your menstrual cycle.

How Does the Calculator Work?

There are several methods for calculating your due date using a pregnancy calculator. Each is effective, but none are 100 percent accurate.

Last Menstrual Period Method

The method is the most common method. Calculators that employ this method add 280 days to the first day of your last menstrual period. This is based on the belief that pregnancy lasts for 266 days--280 minus the estimated 14 days from the start of your menstrual cycle until ovulation. The date of your ovulation is an estimate; many women may ovulate before or after day 14. Because of this uncertainty, the due date calculation is considered an estimate.

Naegele's Rule

Another way to calculate your due is with the Naegele's Rule. If you know the first day of your last menstrual period, the calculation is simple. You count back 3 months from this date and add 7 days. Many doctors use this method in their offices since it can be done quickly using a calendar. This method assumes that your menstrual cycle is average or 28 days in length and that ovulation occurs on day 14.

Basal Body Temperature

Basal body temperature (BBT) is your baseline temperature, which is measured first thing in the morning before you do anything--including getting out of bed. You can measure your BBT every month; typically, there is a temperature elevation during ovulation. If you take the day of ovulation and add 266 days to it, you can determine your delivery date. Some pregnancy sites have charts where you can track this data and calculate the delivery date.

If you know your date of conception, for instance if you underwent a fertility procedure, you can add 266 days to that date.

Calculating Your Due Date Manually

Take the date of the first day of your last normal menstrual period and add 7 days to it. Now, add nine months to get your due date.

What If You Have an Irregular Menstrual Cycle?

Online due date calculators work best when you do have a regular period. It is advisable to discuss your due date with your health care professional, in the event your cycle is irregular. Your OB care provider will suggest the use of ultrasound/sonograms to clarify your possible conception and due date.

Due Dates Are Only Estimates

While due date calculators can give you a general idea of when to expect to birth your baby, there are many factors that may lead to your having your baby before or after that date. Less than ten percent of all pregnancies result in a deliver on estimated due date, so it is just that -- an estimate.

Babies who are born at full term will be born, in general, somewhere between 37 and 40 weeks from the first date of your last period, but not all babies are born at full term. Health concerns, accidents, and multiple births can all play into when your baby will actually be born. The uncertainty is par for the course -- your best plan is to get prepared for the birth early and relax.

Due Date Calculator