How Is Your Estimated Date Of Delivery Calculated?

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A typical pregnancy lasts, on average, 280 days, or 40 weeks, starting with the first day of your last menstrual period (LMP). 

Calculating your due date is not an exact science as very few women actually deliver on their due date. Nevertheless, it is important to have an idea of when your baby will be born. Accurate information about the age of your foetus (unborn baby) is necessary for monitoring its development throughout your pregnancy.


Methods For Calculating Your Due Date

If you have a regular 28-day menstrual cycle, your due date can be estimated using one of two methods. 

The first method calculates the due date by using a simple mathematical calculation based on the start date of your last period. The second method, which is widely used by healthcare professionals, is the pregnancy wheel. This circular wheel can estimate both your due date and provide helpful timeframes for various antenatal checks.

If you have an irregular menstrual cycle or cannot remember the first day of your last period, the aforementioned calculation methods may not be helpful.

In such cases, your doctor will probably need to use ultrasound imaging tests to determine your expected due date. An ultrasound scan can determine the age of your unborn baby (foetus) with a fair amount of accuracy within the first trimester (3 months) of pregnancy.

What Calculation Can I Use To Estimate My Delivery Date?

If you know the first day of your last menstrual period, there are a couple of simple calculations you can do to estimate your baby’s due date.

One method is called Naegele's rule. This simple calculation adds 7 days to the first day of your LMP and then subtracts 3 months. 1 year is then added. For example, if your LMP was 1 November 2022, add 7 days (8 November 2022), then subtract 3 months (8 August 2022). Lastly, add one year (8 August 2023) to calculate the expected delivery date.

There are a number of variations to the Naegele's rule calculation. A common calculation involves counting back 3 calendar months from the first day of your last period, and then adding 1 year and 7 days to that date.


How Does A Pregnancy Wheel Work?

A pregnancy wheel is a very popular method of establishing your due day of delivery. It does, however, require you to remember the first day of your last menstrual period. 

These handy devices are very affordable and can be bought online for under £10. Besides being able to estimate your due date of delivery, a pregnancy wheel also includes important landmarks of pregnancy. It provides timeframes for scheduling various antenatal checks such as heartbeat checks and ultrasound viewing.

A pregnancy wheel is comprised of two interlocking circular pieces of cardboard attached by a fastener. The months and days of the year appear on the outer wheel to calculate due dates. Information on fertility, prenatal screenings, and your baby’s development are located on the inner wheel.

You use a pregnancy wheel by following a few easy steps. To begin, you need to locate the ‘first day of last period’ on the wheel and move the pointer to that date. Once you have done this, all the other important dates in pregnancy will align with the appropriate dates.


Using An Ultrasound To Establish Due Date

If you can’t remember when your last period was or have very irregular periods, your doctor may need to do an ultrasound to determine your due date. 

During the first trimester (before 14 weeks) an ultrasound can help predict the due date based on the crown-rump measurement (CRP) of your baby. This is a straight-line measurement from the top of your baby’s head (crown) to the bottom of their bottom (rump). The CRL estimates foetal age with an error of 3 days from 7-10 weeks and of 5 days from 10-14 weeks of pregnancy.

Once the size of the crown-rump length reaches 84 mm (between 12 – 14 weeks) a different method, called the biparietal method, needs to be used to determine an estimated date of delivery. The biparietal method measures the circumference of the baby’s head to determine its age. Although fairly accurate in the early stages of pregnancy, this method becomes less accurate as the pregnancy advances.

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