How can I calculate my baby's due date?

Knowing exactly how many weeks pregnant you are and figuring out when your due date or estimated delivery date (EDD) is useful. It can help you prepare and plan for life with your new arrival.

To help you understand your pregnancy timeline, let's run through how to calculate pregnancy weeks and a baby's due date. We'll also cover how long a typical pregnancy lasts, and answer some of the most frequently asked questions about pregnancy weeks and due date calculation.

How many weeks are you pregnant?

Pregnancy typically lasts between 37 to 42 weeks. On average, single-baby pregnancies tend to last 39 weeks from conception. For those expecting twins, full-term pregnancy is considered at least 38 weeks.

How to calculate pregnancy weeks and your due date

Although healthcare providers consider a full-term pregnancy to be anywhere between 37 and 42 weeks, it takes around 38 weeks for a baby to fully develop. Based on this 38-week gestation process, your due date is the approximate day you're expected to give birth.

When you're pregnant, you can work backwards from your due date to calculate how many weeks pregnant you are. There's two main methods you can use to calculate your due date - your last menstrual period or the date you conceived (got pregnant).

Last menstrual period method

Your due date is around 280 days or 40 weeks after the day your last period started. This is a good way to figure out when your due date will be if you use an app or diary to track your periods. However, if you have an irregular menstrual cycle, it's not always accurate.

Conception date method

If you know the exact day you conceived, that date can be used to calculate when your baby will arrive. To do this, you can count 38 weeks forward from the day you got pregnant, and that day will be your due date.

Remember though, that this technique can only be accurate if you know for sure when you became pregnant.  You can get pregnant between 12 to 24 hours around the time that you ovulate, but sperm can live inside your fallopian tubes for up to five days. So, if you ovulated a few days after you had sex, you could have potentially become pregnant then.

Don't worry if maths isn't your strong point. Pregnancy Birth & Baby and Clearblue have handy online due date calculators that can crunch the numbers for you!

How accurate is my due date?

During your antenatal appointments, your healthcare provider will examine you and your developing foetus by performing an ultrasound to confirm your due date. That said, the truth is that a calculated due date is only a guide estimate and shouldn't be 100% relied upon.

Only a small percentage of babies are born on their predicted due date.

Will having multiple babies affect my due date?

Usually, most parents find if they're pregnant with multiple babies during their 12-week scan. During this scan, it's usually possible to see if the babies share a placenta and are identical or if they have two separate placentas, meaning they could either be identical or not.

Expecting multiple babies (twins, triplets, or more) can affect your due date. If you're pregnant with multiples, you'll be more likely to deliver before your due date, but you'll be provided with more information about your due date during your scan appointment.  

How is pregnancy calculated if I've had IVF?

Your healthcare provider will calculate your due date and share this with you on your transfer day if you use in vitro fertilization (IVF) to become pregnant.

If you'd like to have an idea of your due date before your pre-booked IVF transfer date, you can follow these steps to calculate your due date:

• If your embryos are going to be transferred five days after your egg retrieval, count ahead 261 days from your designated transfer date.
• If your provider is going to transfer your embryos three days after your egg retrieval, you can count ahead 263 days.