Calculating When My Baby Is Due: How Pregnancy Due Dates Work

Finding out you're pregnant often comes with one immediate question:

When is my baby due?

It sounds like it should be a simple calculation, but pregnancy dating can be surprisingly confusing. You may know when you had a positive pregnancy test, you might know when you ovulated, or you may even know approximately when you conceived – but that's not usually where we start when calculating your pregnancy.

So, how do we actually work out when your baby is due?

How is my pregnancy due date calculated?

Your estimated due date is generally calculated from the first day of your last menstrual period, often referred to as your LMP.

A pregnancy is dated as approximately 40 weeks, or 280 days, from the first day of your last period.

So, the basic calculation is:

First day of your last period + 40 weeks = estimated due date

For example, if the first day of your last period was 14 January, your estimated due date would be around 21 October.

It's important to remember the word estimated. Your due date helps us date and manage your pregnancy; it isn't a prediction of the exact day your baby will arrive.

Why does pregnancy start before I've actually conceived?

This is the part that often causes confusion.

If you have a regular 28-day menstrual cycle, ovulation usually occurs approximately two weeks after the beginning of your last period. Conception occurs around the time of ovulation.

However, pregnancy is traditionally dated from the first day of your last menstrual period, rather than the date of conception.

This means that by the time you miss your period and take a pregnancy test, you may already be considered around four weeks pregnant, even though conception probably occurred approximately two weeks earlier.

It can seem a little odd, but it's the standard way pregnancy gestation is calculated.

What if I don't have a 28-day cycle?

Not everyone ovulates on day 14, and not everyone has a regular 28-day menstrual cycle.

If your periods are irregular, your cycles are particularly long or short, or you don't remember the first day of your last period, calculating your due date from your LMP may not be as accurate.

That's where an early pregnancy ultrasound can be particularly useful.

In early pregnancy, measurements taken on ultrasound can help us assess how far along the pregnancy is and establish the most appropriate estimated due date.

Can my due date change after my first ultrasound?

Sometimes.

You may calculate one date from your last period and then have an ultrasound that suggests your pregnancy is a little further or less advanced than expected.

Often, there's a straightforward explanation. You may simply have ovulated earlier or later in your cycle than you thought.

If there is a meaningful difference between your menstrual dates and the measurements on an early ultrasound, your obstetrician will determine which date is the most appropriate one to use for the pregnancy.

Once we have established a reliable estimated due date, we generally don't keep changing it throughout pregnancy simply because your baby later measures a little bigger or smaller.

What if I conceived through IVF?

Pregnancy dating following IVF is different because we have precise information about the timing of treatment and the age of the embryo at transfer.

Your estimated due date can therefore be calculated using your embryo transfer details rather than relying on the date of your last menstrual period.

Why is getting my due date right important?

Your estimated due date isn't just about knowing approximately when you'll meet your baby.

Accurate pregnancy dating helps us know when particular tests and ultrasounds should be performed, assess whether your baby's growth is appropriate for their gestational age and make clinical decisions as your pregnancy progresses.

Later in pregnancy, gestational age also becomes important when we're discussing the timing of birth.

That's why establishing good pregnancy dates early is useful.

Will my baby actually be born on my due date?

Maybe – but there's a very good chance your baby will have other ideas.

Your due date is an estimated date of delivery, not an appointment.

Babies can arrive before or after their estimated due date, and your obstetrician will monitor both you and your baby as you approach the final weeks of pregnancy.

Rather than focusing too heavily on one particular day, I encourage patients to think of their due date as an important reference point around which we plan and monitor their pregnancy.

I've worked out my due date. What happens next?

If you've recently discovered you're pregnant, congratulations.

For women considering private obstetric care in Melbourne, this is also a good time to start thinking about who you'd like to care for you during your pregnancy and where you'd like to have your baby.

You don't need to wait until you're 12 weeks pregnant to contact an obstetrician. In fact, contacting your preferred obstetrician early means the rooms can confirm availability and guide you regarding your first appointment and any early pregnancy investigations.

At Team Cole, we care for women throughout pregnancy, birth and the first six weeks after their baby is born, with support from our obstetric, midwifery and allied health team.

Dr Stephen Cole is a Melbourne Obstetrician and Specialist in Maternal-Fetal Medicine, caring for both uncomplicated and high-risk pregnancies and delivering at Epworth Freemasons, Frances Perry House and St Vincent's Private Hospital Melbourne.

If you've recently discovered you're pregnant and would like to discuss your pregnancy care, please contact Team Cole.