How Due Dates are Calculated

There are many different ways to calculate your due date. Some techniques are more accurate, than others and unfortunately it is not 100% guaranteed. Your due date is only an estimation of the date you will be having your baby. In fact only very few babies are born on the due date.

Menstrual cycle

For women who have regular menstrual cycles calculating the due day using Nagel’s rule is a often the preferred way of calculating the due date. The calculation involves knowing the first day of your last menstrual cycle, adding one year, then subtracting three months, and adding seven days. This method is only accurate if your menstrual cycle follows a cycle of 28 days. If your menstrual cycle is longer than 28 days, then you will to add to the number of days it is longer by. If your menstrual cycle is typically 30 days, then add 9 days. If you have an irregular menstrual cycle, then this will not be the best method for calculating a due date.

Date Conceived

One of the most accurate ways of calculating a due date is knowing when conception occurred. This method is more accurate for the individual who knows when they ovulated and had intercourse.

Pregnancy Wheel

A pregnancy wheel or OB wheel is based on the average number of days of a pregnancy cycle of 280 days or 40 weeks. This is also based on an average dates and the companies the produce and make the wheels may choose a different average number of days. This means that if you calculate your due date with 3 different pregnancy wheels, you could end up with 3 different dates. Often, it is seen that these dates differ only by 1 or 2 days.

Ultrasound Due Date

Ultrasounds can also detect due dates based on the development of the embryo. The sooner an ultrasound is performed, the more accurate it is at estimating the due date. In the first trimester ultrasounds can estimate the due date and is accurate within a few days. Having an ultrasound later in your pregnancy is less accurate and may provide you with an estimate, which may be off by 2 weeks.

Online and electronic due date calculators

Online due date calculators and phone apps are based on the same method of putting the date of your last menstrual cycle, adding 280 days and it equals the approximate due date.

Consult with your healthcare provider

Selecting which method to choose your due date is best to have with your healthcare provider.  Healthcare providers will consider all factors that impact the due date and can often take into consideration due dates. Often times healthcare providers will prefer to use the technique based on the last menstrual cycle, even if the US revealed a different date. Often healthcare providers will prefer to use the menstrual cycle calculation if it is within 6-10 days of the US date. If the US date is different than more than 6-10 days then the US date will be used.

Find out how to know when you are ovulating