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Here’s How You Can Calculate Your Due Date and Conception Date

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If you’ve confirmed that you’re pregnant, you most likely want to know when you conceived your baby and when you’ll give birth. Knowing these dates is important since they’ll give you an idea of how far along you are and help you mentally prepare for the pregnancy milestones that you’ll soon experience. Of course, they’ll give you plenty of time to get ready for your baby’s arrival and ensure you’re well-equipped to welcome him to the world.

Take note, though: it’s usually not possible to calculate the exact
conception date and due date. What you can do (and what most doctors do as well) is to estimate the date when you conceived your baby and use it to estimate the date when you’ll give birth. Experts point out that only around 5 percent of babies are born on their estimated due date, so don’t put too much emphasis on any specific date. You can give birth before or after the date that you calculate!

Ways to Calculate

Despite the fact that you can only make estimates, it’s still essential to have an idea of your conception date and due date. Here are some of the methods you can use to calculate them:

Manual computation

Get a pen and some paper and write down the first day of your last menstrual period (also known as LMP). If you have a regular 28-day menstrual cycle, add 12 to 16 days to this date to determine the days when you were ovulating and had the chance to conceive. For instance, if the first day of your last period was January 01, 2017, adding 12 to 16 days to it results to January 13 to 17 ” these could be your conception date.

To calculate your due date, you simply have to add 280 days or 40 weeks to your last menstrual period. Remember: 40 weeks is just an estimate; your baby might come out 38 weeks to 42 weeks after your LMP.

Online calculators

To save time and get a more accurate estimate, you’ll want to use an online pregnancy calculator like the one offered at menstrual-cycle-calculator.com. Online calculators include the average length of your menstrual cycle in the equation, instead of just basing it on the default variable of 28 days. This means they can determine the dates when you ovulated in a more accurate way and provide a more precise estimate of your conception and due dates.


Your OB-GYN can use your ultrasound results to measure your baby’s size which, in turn, can help determine his gestational age as well as the date when he would be delivered. However, this method works only in the early part of pregnancy, specifically between the 8th and 18th weeks. After that, it becomes harder to estimate your baby’s gestational age simply through his size.

Factors That Affect Calculation

As mentioned above, you must know your LMP as well as your average menstrual cycle length when calculating your conception date and due date. However, these are not the only things to consider since the following factors can also affect your calculation:

  • LMP awareness ” If you can’t remember the first day of your last menstrual period, it’s virtually impossible to know when you conceived and when you might give birth.

  • Regularity of menstrual cycle ” If you have an irregular menstrual cycle (meaning your period comes earlier or later than expected or if you often miss a period), it will be more difficult to calculate your conception and due dates.

  • Growth of your baby ” Some fetuses are larger or smaller than normal. In these cases, it can be difficult to measure their gestational age based on their size.

  • Method of fertilization ” If you conceive your baby through in-vitro fertilization and other procedures, you’ll be able to know your exact conception date.

Consult Your Doctor

Calculating your conception date and due date on your own can be fun and interesting. But, for more accurate estimates, it’s always advisable to get your doctor’s opinion.