Wait, is pregnancy actually 9 or 10 months?

It’s commonly held that human pregnancy lasts nine months, but then you fall pregnant and you do the math and suddenly find yourself very confused.

Well, obviously every pregnancy is different and babies can be born well before (and after) their due date...

Typically, pregnancy is measured as 40 weeks (280 days) and broken into three trimesters of approximately 13 weeks each, so it’s not quite as simple as saying 9 months. 

So what are the three trimesters?

• 1st trimester = weeks 1-13
• 2nd trimester = weeks 14-27
• 3rd trimester = weeks 28 until 41 (or whenever your baby arrives). 

‘Full term’ is when the baby is considered fully formed and would survive in the outside world and is generally counted from week 39 (weeks 37-38 are called ‘early term’). Beyond week 42 the placenta starts to become unviable and poses a risk to the mother and baby. 

So when you factor in a month(ish) of being pregnant before you even realise it, and then the potential to go overdue it’s more like 10 months in the end.

So how do you work out your due date?

The first week of your pregnancy is actually counted as your last menstrual cycle (so before conception even actually takes place – just to confuse you even more!) and so your expected due date will be counted as 40 weeks from the first day of your last period. 

Alternatively, if you know the exact date you conceived you can then add 38 weeks on this. 

But don’t worry if you can’t remember when any of these dates were, when you have a scan your doctor will be able to gauge your due date. This tends to be more accurate as every woman’s cycle is different and the day in which she ovulates – and the egg is fertilized – will vary. 

Wait, what is the fourth trimester?

The fourth trimester you ask? Don’t worry its not another hidden trimester that you’ll be pregnant for, this is actually what is referred to as the period from when your baby is born to when they are three months old.

While it might seem odd to still refer to it as a ‘trimester’ when the baby is no longer in the whom it is a way to understand the transition that the baby is going through now on the outside and acknowledge that it is still a period of major growth and development.