Nobody wishes to wake up to labour. Although, sometimes, it can occur if you have a cryptic pregnancy where there are only mild signs of pregnancy until labour begins. 

Labour is something you need to be prepared for mentally, physically and spiritually - as the case may be. And an online pregnancy calculator test helps you to do that.

If you are wondering what to do after taking a pregnancy test and it shows positive, a due date estimator or calculator will help.

In this read, we’ll focus extensively on the pregnancy calculator, looking at what it is and how it works.

What is a Pregnancy Calculator?

Usually, a pregnancy lasts 38 weeks from conception or 40 weeks from the last menstruation. According to the World Health Organization, a typical pregnancy lasts between 37 and 42 weeks. A pregnancy calculator is a plan that helps you ascertain your Expected Date of Delivery (EDD). 

How does a Pregnancy Calculator work?

It works by calculating pregnancy based on the given due date, last menstrual period (LMP), ultrasound date, conception date, or IVF transfer date.

Although there are several methods you can use to calculate your pregnancy, if you have a regular period, it is much easier.

Methods of Calculating Pregnancy

  1. Naegele’s rule

Naegele’s rule is a simple method which was first described in 1806 by Franz Karl Naegele, a German obstetrician. Typically, what we know as the most common method is adding 40 weeks to the first day of your LMP, but Naegele’s rule is a much simpler method:

“Add seven days to the first day of your last period, and then count back three months.”

For example, if the first day of your last menstrual period is 2nd September 2022.

  1. Add 7 days = 9th September, 2022
  2. Subtract three (3) months = 9th June 2022

You can’t give birth in the past, lol. So, in this case, you would change the year to 2023. And there you have it!

However, it's important to note that this method works with a typical menstrual cycle of 28 days, as it's common for your period to last between 21 and 45 days

If your cycle is longer, your predicted due date will probably be later if your cycle lasts longer, and if your cycle is shorter, your estimated date might be earlier. 

  1. Period Wheel

Source: Due date Calculator - Flo

Using a pregnancy wheel is another method of determining your due date. If you have access to a pregnancy wheel, estimating when you will give birth is extremely simple.

First, you start by finding your LMP on the wheel. After this, the wheel shows your due date when you align that day with the indicator.

Keep in mind that the due date is merely a rough prediction of when you will give birth. The likelihood of having a baby on that specific day is quite slim. About 80% of the time, delivery occurs between two weeks before or after the due date.

Other methods are:

  1. Mittendorf-Williams rule
  2. Parikh’s rule
  3. Wood’s rule

What If I don’t know my LMP?

Don't feel left behind if you don't know or can't remember your LMP. You'd be surprised most people don't know too. But there are ways you could still figure out your due date without your LMP: 

  1. Ultrasound scan: You could do an ultrasound scan to know your due date. In the early stages of your pregnancy (first trimester), Ultrasound could also be used to confirm the calculation based on LMP. 

If you have irregular periods, you could also use this method. 

During the first trimester of pregnancy, ultrasound measures are more accurate for determining the age of a fetus. However, these ultrasound measures are no more accurate in the later stages as the fetal growth rate varies from pregnancy to pregnancy.

  1. Your doctor can also help estimate your due date if you don’t know your LMP.

How accurate is a Pregnancy Calculator?

Pregnancy calculators are just estimates. If your calculation lands on 25th December - before you get excited about having a Christmas baby - note that the dates could change depending on various factors. 

  • Firstly, the estimated date you get from your calculations using the methods mentioned or an Ultrasound is a middle ground. Usually, births occur within two (2) weeks before or after that date.
  • After your doctor performs an ultrasound, the pregnancy report usually includes two estimated dates. One is based on calculations using the LMP, and the other is based on the ultrasound - these dates are hardly the same. Your doctor will decide whether or not these dates agree after assessing the ultrasound results. If the dates are far apart, your doctor might likely revise your due date.

Online Pregnancy Calculators

Here are a few websites and apps you could use to calculate your pregnancy and due dates. Most of these calculators work with the date of the first day of your last menstrual period.

  1. Flo

Flo is a health app you can use to track your periods, predict your cycles, and learn how to get ready for conception, pregnancy, early parenthood, and menopause. 

It is available on Google Play Store and Apple Store

  1. Online Pregnancy Calculator - Pregnancy Help Online
  2. Pregnancy calculator - Find pregnancy due date, weeks, trimester
  3. Pregnancy Calculator
  4. When is the best time to take your FIRST RESPONSE™ Pregnancy Test?

Written By:
Shalom Ngbala-Okpab
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Further Reading