How to Calculate Your Due Date


Working out when you’re due sometimes feels like rocket science and can be a bit overwhelming. Is it 9 months from your positive pregnancy test? Is it 40 weeks? Is it from the date of your last period? Is it from the date you had sex? Just sit back, take a deep breath, we’ll help you work out when your baby might be entering this world.

So that you have a benchmark in place to measure the growth and development of your baby, pregnancy is said to be 40 weeks long, even if it isn’t (only about 50% of first-time mums give birth in their 40th week). A full-term baby can be born between 37 and 42-weeks’ gestation. Over 41-weeks is classed as overdue but isn't abnormal.

Annoyingly, the day ‘X’ from which we start counting 40 weeks isn’t from the night you conceived, nor from the day you ovulated. 40 weeks is counted from the first day of your last menstrual period (LMP).

Yep, that means any sign of blood, whether it’s spotting or full on 'that time of the month', that’s your day 1 of pregnancy, even if you don’t fall pregnant for another 2 weeks.

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So, let’s do the maths, say your period started on 4th October, you had sex and conceived on the 19th of that month, and you took your pregnancy test on 30th. Ignore every other date other than when you started your last period and count 40 weeks from that date. Your baby’s due date in this scenario, will therefore be around the second week of July the following year (around 9th-11th depending on your period cycle length). You can also calculate it by subtracting 3 months from your LMP and add 7 days. You should arrive on the same date.

Of course, this means by the time the sperm meets the egg and cells start multiplying, you’re already in week 2 of pregnancy, and you’ll be in week 4 by the time you miss your next period and discover you're pregnant.

We know it doesn’t make much sense, but it is what it is.

When you have your first scan between 12-14 weeks, which measures the size of the embryo/fetus, the sonographer might give you a slightly different date, but just remember, the due date is always an estimate. Only 4% of babies are born on their estimated due date19. Ultimately, your baby will be the one that tells you they’re done cooking, we just must do all we can to prepare for that day.

So, if you are calculating your due date you probably have a heap of other questions! Need to speak to a midwife? We have just the answer: Early Pregnancy Workshop with The Honest Midwife's FREE

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