Estimated Due Date: How Do You Calculate It?

Now that you have done your test and your blood test, and you are sure you are pregnant, you are asking yourself THE big question: when will I give birth? Your EDD (Estimated Due Date) will give you the answer! But what exactly is it? How is it calculated? Is it reliable? We take a closer look in our article.

What is the Estimated Due Date?

At your first pregnancy appointment, your gynaecologist or midwife will talk to you about your EDD and estimate it. To do this, your healthcare professional will use a “pregnancy wheel”. Based on the date of your last period, they will estimate your expected date of delivery, also called your EDD.

As you will have gathered, this date is “estimated”. It is therefore a purely theoretical date, even though it is estimated by your doctor. The idea is to have an approximate date for the end of your pregnancy, and therefore for the birth, based on your estimated date of the start of pregnancy (known in France as the DDG). This simply corresponds to the day your egg was fertilised.

How do I calculate my EDD?

While a pregnancy theoretically lasts nine months, or 41 weeks since your last period, the reality is a little more complicated. To date the day of delivery accurately, you need to be sure of the date the pregnancy started and take into account each woman’s individual characteristics. There are several ways to estimate your EDD.

First, you can calculate it using the date of conception. To do this, simply add 9 months to that date and you will have your EDD. However, this method is not the most common, as it is difficult to pinpoint your ovulation date precisely.

The most widespread way to calculate your due date is based on the first day of your last period. Your doctor will mark this date on their “pregnancy wheel” and will then estimate your ovulation date, which is on average 14 days later. In France (the average differs from country to country), 9 months are added to the ovulation date to calculate the Estimated Due Date. So a woman whose last period started on 4 February will ovulate 14 days later, on 18 February. Her EDD is therefore estimated at 18 November.

Another method: the dating scan

All these calculations are based on averages. But not every woman ovulates on the 14th day of her cycle. That is why your first scan will allow you to calculate your due date more accurately. Also known as the dating scan, this first scan is carried out between the 11th and 13th week since your last period.

This scan will allow your doctor to calculate precisely when your pregnancy started (which corresponds to your ovulation) and so estimate your due date more accurately.

To do this, the practitioner will measure the foetus from head to bottom (the crown-rump length). They will then compare it with a reference curve. In this way, they will estimate the age of the foetus and the date of conception more accurately. By adding 9 months to this date, you will then get your estimated due date.

Can I rely on my EDD?

As mentioned earlier, the EDD is theoretical and therefore approximate. Why? Few babies are born exactly 9 months to the day into the pregnancy, and the calculations can be thrown off.

If the EDD is calculated from the first day of your last period, the margin of error is considerable. Various factors can affect the date of fertilisation. The calculation is based on a 28-day menstrual cycle with ovulation on the 14th day. But in reality, the length of the cycle and the date of ovulation can differ for each woman. What’s more, even if the egg is released on the expected date, fertilisation may happen later. An incorrect date for the start of your last period, or bleeding that could be mistaken for a period, are also factors that come into play. These can also lead to errors when calculating the estimated due date.

However, by combining this estimated date with the dating scan, your EDD will become more precise and closer to reality. Indeed, in 95% of cases, measuring the foetus from head to bottom gives an accurate pregnancy start date to within plus or minus 5 days.

Even so, you still can’t rely 100% on this EDD. Your little one may well make an appearance before or after that day.

Why is it important to know my EDD?

Knowing your due date is useful both medically and administratively. Even though it’s only an indication and isn’t entirely reliable, it’s used as a reference point.

First, your EDD plays a key role in organising your pregnancy follow-up. Your doctor uses this estimate to tailor your care and, in particular, to schedule the dates of your three scans. This date also marks the end of your pregnancy. If necessary, it can therefore be used to decide when to induce labour to avoid any risk of complications. In addition, on the administrative side, your maternity leave in France is calculated from your EDD (6 weeks before your EDD and 10 weeks after the birth for a standard pregnancy). Finally, it is used to estimate the leave your partner is entitled to.

Despite all these calculations and estimates, your Estimated Due Date is above all the day you get to meet your little one! It gives you and your partner the chance to get ready to become parents and to organise baby’s arrival!

Dylan
Proud dad of three little ones and founder of Premiers Moments. Together with the whole team, I share our parenting tips here to support you, from pregnancy through your child's first years.