Free Conception Calculator : Your Due Date

A conception calculator, sometimes known as a reverse due date calculator, can give you a rough idea of when your baby was conceived. Some couples are keen to know when they struck gold, and our conception calculator can help pinpoint the moment you got pregnant.

What is a pregnancy conception calculator?

A conception calculator is a simple and effective tool that determines the due date of conception. This can be useful for expecting parents who are desperate to know more about when their baby was conceived and the early stages of pregnancy. The conception calculator will help you to pinpoint the right time by using information like the first day of your last period (LMP), or the due date given by to you by your doctor. With this, we can estimate the time frame in which conception took place.

You can try out BellyBelly’s Pregnancy Conception Calculator.

What is a conception date?

The conception date is the date your baby was conceived. Although you might think this means the day you had sex, the day intercourse happened isn’t necessarily the day sperm met egg. Did you know sperm can survive inside the female reproductive tract for up to five days?

How do I calculate when my baby was conceived?

If you’re desperate to know when your little one was conceived, you can try to calculate the potential date of conception. To do this, you’ll need to know the date of the first day of your last period. Assuming you have pretty regular cycles, your baby was probably conceived between 11 and 21 days after the first day of your last period.

If you aren’t good with numbers, save yourself some tricky maths and let the BellyBelly due date calculator do the work for you.

How does a pregnancy conception calculator work?

The BellyBelly conception date calculator works by analysing your data and determining the most likely date of conception. You can choose to enter either your baby’s due date, the date of your last menstrual period (LMP), or your baby’s gestational age at the dating scan or early ultrasound.

The conception calculator will perform impressively fast backward calculations to determine when conception is likely to have occurred.

Understanding your fertile window

When we refer to calculating your due date by using the conception date, people often assume we’re talking about the day you had sex that resulted in pregnancy. If you have intercourse at the time, or soon after ovulation, the sperm will find the egg so fertilisation can occur. However, if intercourse happens in the days leading up to ovulation, the sperm can lie in wait for up to five days inside the reproductive tract.

Based on the calculation method used for conception date, you might think it’s wrong, but if you had sex anytime within your fertile window, then you could have conceived at the time of ovulation. Your fertile window begins about five days before ovulation and ends 12-24 hours after ovulation has occurred.

Understanding and tracking your basal body temperature (BBT) may help you identify your fertility window, as it rises slightly after ovulation.

How does conception occur?

Conception occurs when a sperm fertilises an egg. During ovulation, a mature egg is released from the ovary and makes its way down the Fallopian tube towards the uterus. Fertilisation typically occurs while the egg is making the journey down the Fallopian tube.

Although conception usually occurs around the middle of your cycle, you won’t find out you’re pregnant until your next period is due. Home pregnancy tests are most accurate when used from the first day of your missed period.

How accurate is a pregnancy conception calculator?

The accuracy of the conception and due date calculator is determined by the information you put in. The information you choose to provide to the pregnancy calculator is used to determine the date calculated for you.
There are other methods to try:

Estimated due date

Before you receive your baby’s official due date at the ultrasound scan, your doctor will give you an estimated due date in early pregnancy. There is no such thing as an exact due date, so there might be a date change at some point; remember, it is only an estimate. The early estimated due date is based on the dates of your last menstrual period (LMP) and these calculations are often inaccurate, especially for women with irregular cycles.

The calculations used to determine the estimated due date assume all women have a 28-day cycle. A 2020 study published in the Journal of Internet Medical Research found that only 16% of participants had a 28-day cycle.

For women who have irregular periods, with cycle lengths that vary month to month, it can be challenging to determine your average menstrual cycle length. Your pregnancy due date, especially the one estimated before your ultrasound date, is unlikely to be spot on.

If you don’t have an estimated pregnancy due date yet and would like to calculate your due date, BellyBelly has a Great Pregnancy Due Date Calculator you can try.

Last menstrual period (LMP)

If you don’t yet have an estimated due date, you can calculate your predicted conception date using the date of your last menstrual period. For this, just enter the date of the first day of your last menstrual period (LMP). The calculator will then assume you conceived around two weeks later, in line with the typical 28-day cycle.

As we’ve seen, though, most women do not have a 28-day cycle length, and so most pregnancies will have due dates that vary, too.

Ultrasound scan

Another option is to use your ultrasound information. In this instance, you put in the date of your scan and your baby’s gestational age at the time of the scan. Although an estimated due date calculation provided by an ultrasound is considered more accurate than one calculated with the date of the last period, it is still not a perfect science.

The first trimester ultrasound is often referred to as a dating scan. A 2013 study published in the International Journal of Obstetrics and Gynaecology found that ultrasounds performed after 20 weeks were less accurate than dating scans performed between 11 and 14 weeks of pregnancy.

A 2018 study into IVF pregnancies where the date of fertilisation is known, published by the British Medical Ultrasound Society, found that the use of scans often led to the overestimation of gestational age (and therefore the pregnancy due date) by around three days. Although it’s not perfect, entering information from your baby’s dating scan is more likely to give accurate conception date results.

Am I pregnant the day I conceive?

This is a tricky one. Pregnancy is officially considered to begin two weeks before ovulation. Generally, this is expected to occur around the time of your last menstrual period. However, you won’t really know until you miss your next period. That means when you track, you will have been two weeks pregnant at the time of conception (confusing, right?).

At the time of conception, you’d have no idea you were pregnant. There are no obvious changes or signs at the time of conception, although some pregnant women claim they had ‘a feeling’ that something was different.

Can I use the conception date calculator to plan when to conceive my baby?

Technically, you can use the conception calculator to figure out when you would have to conceive to have a baby at a particular time of year. It’s worth bearing in mind that this could end up being inaccurate.

First of all, you might not fall pregnant. Only a third of couples become pregnant in the first month. Also a full term pregnancy is considered to be anything between 37 and 42 weeks; this makes it difficult to pinpoint an exact day. Finally, you would be relying on your cycles matching up with your intended conception date, which could be unlikely.