Mother and Baby

How many weeks pregnant am I? Here's how to count in weeks and months

Section: Pregnancy
How many weeks pregnant am I?

We all know, as much as you might try and work out your date of conception, your little one is going to arrive when it wants to. That said, when it comes to scans and appointments, you’ll need to have as accurate an idea as possible on how many weeks pregnant you are. To help you with the pregnancy math, we’ve come up with this (hopefully simplified) guide.

How many weeks pregnant am I?

It’s tricky to be completely accurate when working out the exact day you conceived your baby. As a rule, we usually say a woman is ovulating around two weeks after the first day of her last period, and her egg will be fertilised around 24-36 hours after ovulation.

Why do midwives go off the last period?

In your first appointment, your GP or midwife will ask you all about your last period. When did it start? When did it end? Was it lighter than normal? From this they can work out whether it was actually a period, or an implantation bleed, which some women experience. (You can find out everything you need to know about implantation bleeds here).

Most medical professionals will use the first day of your last period as the start of your pregnancy, however in reality, the day your baby was conceived is usually two weeks later. It sounds confusing doesn’t it, but this is all because it’s hard to say exactly when we’re ovulating, and which of the days in that window you actually conceived.

Of course, if you have irregular periods, your cycle varies in length or you don’t know the exact date your last period started, things can get a bit more complicated. Most due date calculators (including ours) work best if you do know this date, but don’t panic if you can’t work it out. 

Your first ultrasound scan is often called your ‘dating scan’. This happens at around 8-14 weeks and is used to see how far along you are in your pregnancy. From this, your doctor and midwife will be able to work out a more precise due date as well as making sure your baby is developing correctly. Your sonographer will measure the length of your baby from his head to his bottom, which helps them to work out how old your baby is.

What is the ‘gestational age’?

When we go off a woman’s period start date, technically her baby is always two weeks younger. The length of time between the start of your last period and the actual date you conceived is sometimes called the ‘gestational age’. So, when your doctor tells you you’re four weeks pregnant, your baby’s gestational age is actually two weeks.

How to measure pregnancy in weeks

Here’s where it gets that bit more confusing. In your fifth week of pregnancy, you will still be four weeks pregnant. To help try and get your head round this, let’s think about when your little one celebrates his first birthday, and then is ‘age one’ throughout his second year of life. After being pregnant for seven days, you will be one week pregnant, but you will stay one week pregnant throughout the second week, until you reach 14 days and technically complete your second week of pregnancy.

This means, in your first week of being pregnant, you are technically zero weeks pregnant. In your second week, you are one week pregnant. In your third week, you are two weeks pregnant and so on.

How do I know what trimester I am in?

Trimester means ‘three months’, so your pregnancy will be divided into three trimesters. Your first trimester will be from conception to thirteen weeks and six days. Your second trimester is from 14 weeks to 27 weeks and six days. And your third and final semester is from 28 weeks to when you have your baby (which is normally around 40 weeks from the first day of your last period).

How accurate is my due date?

In two words – not very. With all the will in the world, only about five per cent of babies are actually born on their due date. Most babies will be born between 37 and 42 weeks of pregnancy, so it’s actually very unlikely you’ll give birth on the exact date that has been predicted.

What can I expect in each week of being pregnant?

Pregnancy can be a physical and emotional ride. Luckily for you, we have a week by week pregnancy guide here, which will give you everything you need to know, from week one to 40. 

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