Pregnancy

• You can calculate your baby’s due date by working out your last menstrual period or the date you conceived.
• Your due date is around 280 days or 40 weeks after the day your last period started.
• On the other hand, you can count 38 weeks forward from the day you got pregnant, and that day will be your due date. 

Knowing exactly how many weeks pregnant you are and figuring out when your due date or estimated delivery date (EDD) is can be massively helpful.

With this knowledge, you can better prepare yourself 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. 

For 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 

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. And when you're pregnant, you can work backwards from your due date to calculate how many weeks pregnant you are. 

What is gestational age and how is it related to due date?

Gestational age is the number of weeks and days of pregnancy counted from the first day of your last menstrual period, which is then used to work out the due date that’s typically set at 40 weeks' gestation. 

How to work out your baby’s due date

To calculate your baby’s due date, there are two main methods you can use - 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, 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’ve potentially become pregnant then. 

Don't worry if maths isn't your strong point. The NHS and Tommy's have handy online baby due date calculators that can crunch the numbers for you!  

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. 

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 if this is the case.   

What are the signs my baby is coming before their due date?

There are a few signs that your baby could be coming before their due date, and these include: 

• Lower back pain or pressure 

• Pelvic pressure 

• Diarrhoea or nausea closer to labour 

• Losing your mucus plug 

• Irregular contractions 

And then there’s the usual sign of your water breaking. Read more about early signs of labour here.

Can due dates change during pregnancy?

Yes, it’s very possible that your due date can change, especially during the early stages of pregnancy if the ultrasound shows that your baby’s size doesn’t match what was originally calculated. 

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 5 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. 

Working out your baby’s due date can be fun and exciting, and also a really great way of knowing how much time you have to physically and mentally prepare for your life as a parent.

If you found this article helpful, discover our guide on pregnancy trimesters.