It’s commonly held that human pregnancy lasts nine months, but then you fall pregnant and you do the maths and suddenly find yourself very confused.
Well, obviously every pregnancy is different and babies can be born well before (and after) their due date...
So, is pregnancy nine or ten months? The short answer – it nudges ten. 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 it’ll be 9 months until that super exciting due date.
So what are the three trimesters?
We’re all pretty familiar with the pregnancy trimesters, but how many weeks pregnant are you in them?
First trimester = weeks 1-13
Second trimester = weeks 14-27
Third trimester = weeks 28 until 41 (or whenever your baby arrives).
‘Full term’ is when the baby is considered fully formed in a healthy pregnancy 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 most pregnancies are like 10 months in the end.
Pregnancy months to weeks chart
Here’s a nifty little rundown of what a full term pregnancy should look like from months to weeks.
Month 1: Weeks 1 to 4
Month 2: Weeks 5 to 8
Month 3: Weeks 9 to 13
Month 4: Weeks 14 to 17
Month 5: Weeks 18 to 22
Month 6: Weeks 23 to 27
Month 7: Weeks 28 to 31
Month 8: Weeks 32 to 35
Month 9: Weeks 36 to 40
So how do you work out your due date?
The first week of your pregnancy is actually counted as your last menstrual period (so before conception even actually takes place – just to confuse you even more!) and your estimated due date for a full term pregnancy will be counted as 40 weeks from the first day of your last period.
So is pregnancy 9 or 10 months? With all that in mind, a full term pregnancy more likely hits the tenth month , despite the commonly believed nine month due date.
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 fertilised – will vary.
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.