Calculating Due Date during pregnancy

Photo of author
Written By Brie

After getting a positive on your pregnancy test one of the many things couples wonder is how to start calculating due date(s).

The question “when am I due” is fairly simple to calculate, when you see your Doctor or Midwife for the first time they will ask you when the first day of your last period was and likely use a tool that looks a lot like a wheel to give you a date.

However, this date is not necessarily the day that you will be holding your sweet little one in your arms. This date used is just an estimate, NOT a guarantee! And while many women deliver within a week of this date not all will.

This is why many have started to call their “due date” an “estimated date” or “guess date” or even only been telling people the month the baby is due in.

However, if you are looking to calculate your guess date here are a few methods commonly used.

First Day of Your Last Period Method for Calculating Due Date(s)

This is the most commonly used method and often the one used by care providers to provide you with a date. It relies on the fact that:

A) the average length of your menstrual cycle is 28 days.

B) most pregnancies are 38 weeks from conception

C) Typically a woman will conceive/ovulate 2 weeks after the first day of her last period.

So if you know the first day of your last period was, add 280 days to it to get your due date (280 days = 40 weeks).


Provided by

You could also do the subtraction method which means you take the first day of your last period and subtract 3 months then add 7 days.

For example, if the first day of your last period was May 15 you would subtract 3 months to get February 15, then add 7 days to give you your final date of February 22.

While these are both pretty solid ways to calculate your date if you have a longer or shorter average cycle it may be less accurate for you.

Either way, always keep in mind that it is totally normal to NOT deliver on the baby’s guess date.

Conception Date and Calculating Due Date

Calculating a due date based on your period works for those of us with relatively consistent menstrual cycles. But as mentioned above it may not always be as accurate for those of us with irregular cycles.

Care providers rely on the estimated date of delivery for many aspects of your pregnancy so it is important to tell them if you experience irregular cycles so they can calculate your estimated due date in a different method.

If you have been tracking your cycle and sexual activity it may be just as easy to calculate your due date from that by adding 266 days.

IVF Transfer Date When Calculating Due Date

There are thousands of babies born every year from a variety of fertility treatments. In the US the CDC estimates that over 77,000 babies are conceived from IVF alone.

However, because this is not part of a woman’s regular rhythms calculating from the first day of your last period won’t be accurate.

So just use your IFV Transfer (aka conception date) instead.

Ultrasound Dating Scan

So I totally get how many people don’t track their cycle, may not remember when they had sex or had sex often enough to not know which day conception occurred. Or for many other reasons may not know the information needed to calculate an accurate Due Date.

In that case, you can use other clues to calculate a due date at your first prenatal appointment.

An Early Ultrasound

Not all women get an early ultrasound, some care providers perform them routinely others only offer them under certain circumstances like irregular periods, maternal age of 35+, history of miscarriage, history of pregnancy complications etc.

During the Ultrasound the technician will measure the baby and provide a calculated due date based on the baby’s size.

Pregnancy Milestones

Things like hearing babies heartbeat is usually around week 9 or 10. When you first feel baby move typically happens between weeks 18 to 22. Milestones like this can provide information on where in the pregnancy you and baby are.

It is worth it to note though that these milestones can have some variation and be either earlier or later than the average weeks listed.

Fundal Height

Fundal Height is the measurement from your pubic bone to the top of your uterus. Most care providers will measure this at each of your appointments though some don’t do so until part way into the 2nd trimester.

Uterine Size

In the early stages of pregnancy, some practitioners will perform an internal exam to assess the health and condition of you and baby. If this was done then the size of your uterus can be used to calculate a due date.

Planning your Due Date…

I know when we were trying to conceive I tried to plan my due date or more accurately due MONTH. And while it is possible to get pregnant during a particular month you likely won’t be able to map out exactly when you’ll give birth to the day (or even the week or month).

Babies do tend to arrive when they choose after all…

However, if you are trying to avoid being heavily pregnant during the summer heat or would like to deliver in spring, you could well get lucky and conceive in the right time to do that.

Why Due Dates Change

On occasion, a care provider will change a calculated due date. There can be a number of reasons for this and usually is nothing to worry about.

Before my first pregnancy, I was contending with an irregular cycle so the original due date was calculated based on that. We realized later that I had mistakenly counted some implantation bleeding as part of my normal wonky cycle.

A couple of months in we figured I was further along because of the ultrasound measurements showing a rather larger than expected baby…so the date was adjusted.

Make sure you talk to your Provider about the reasoning and ask questions to ensure you understand what is happening and why.

All in all you don’t need to wait for your Care Provider to calculate your Due Date, but it is a good idea to confirm it with them before announcing the date to the world.