Pregnancy is technically calculated in weeks, starting from the first day of your last menstrual period. However, when chatting about our pregnancy with friends and family, we usually say 'I’m 5 months pregnant and not 'I’m 22 weeks'.
The correlation between weeks and pregnancy can often confuse mums-to-be. Here's a guide on how to work out how far pregnant you are!
How to calculate pregnancy weeks
You’ll soon discover that gynaecologists, midwives, healthcare professionals always use weeks, not months, when referring to your pregnancy.
Why are weeks and not months used when talking about how far pregnant I am? Is it just to complicate our lives? As unlikely as it seems, there’s a valid explanation: months have a variable duration, whereas weeks are always composed of 7 days and, therefore, are a more stable and easily measurable reference.
A pregnancy lasts about 40 weeks, and this is used as a basis when we work out due date. This is an average, as babies decide to come into the world anytime between 2 weeks before and 2 weeks after your due date.
If, for example, your due date is 9th June, your baby could be born anytime between 25th May and 24th June.
This, of course, is true if you actually reach the end of the standard gestation period, i.e. between the 37th and 42nd week. If your baby is born before 37 weeks, they will be considered premature, and your pregnancy will be called preterm.
Why do I use the date of my last menstrual cycle to calculate how many weeks pregnant am I?
To calculate how many weeks are you pregnant, you need to start from the first day of your last period. Why is that if actual conception occurs at least 15 days afterwards?
It’s quite simple: because it is the only accurate reference that we have. Ovulation usually occurs 14 days after the beginning of your menstrual cycle, but this is not always the case, especially with irregular cycles. The precise time of ovulation is therefore not easy to identify, whereas we usually always remember the day we started our last period, especially if we’re trying to conceive.
This reference will serve as a starting point if you’re wondering when is my due date? - if your cycle is regular and ovulation occurred around day 14 of your cycle, you’ll see that your due date will be 40 weeks from then.
Otherwise, it will be up to your gynaecologist or midwife to determine whether the birth should be pushed back if ovulation occurred at a later date.
Pregnancy due date calculator with irregular menstrual cycles
If your cycles last longer than the standard 28 days or are irregular, it’s trickier to work out how many weeks are you pregnant and to try to coincide your last menstrual cycle with actual gestation.
During your first ultrasound scan, referred to as NHS dating scan which usually takes place around 10 to 14 weeks of pregnancy, your doctor should be able to make a good approximation of your baby’s gestational age, which can, however, always be corrected along the way if necessary.
Let’s look at an example: assuming your last period was on 26th August, and you go to your doctor in what you believe to be week 7, i.e. around 14th October, and then your doctor or midwife notes from your pregnancy ultrasound scan that your baby is smaller than they should be if you ovulated on day 14 of your cycle. What will happen? Your healthcare specialist will identify a tentative date for the start of your last menstrual cycle.
Let's assume, for example, that your baby is 'one week behind'. The date of your last period will be 'shifted' to 2nd September, thus pushing your due date back by one week.
Why is pregnancy calculated in weeks?
First of all, it allows mums-to-be to measure the progress of their pregnancy and to have an idea of their due date.
In addition, from a medical perspective, it is important for monitoring fetal development and thus the progress of the pregnancy itself. It helps to understand whether the foetus is developing regularly, according to standard reference parameters (e.g. femur length).
If these parameters are not met and the healthcare professional notices that the baby is smaller than it should be, pregnancy could be delayed, and the due date pushed back.
Not only weeks, but also days
If you have downloaded an app that helps you count how many weeks are you pregnant, you will see that it also accurately counts days. This serves to make the pregnancy calculator as accurate as possible.
Be careful though: let's say for example that today is 20 + 5 weeks for you.
Being 20 + 5 weeks pregnant means that the 20th week has already ended, and you are actually in your 21st week. Which will end at 21 + 0 weeks.
How to work out how many months pregnant you are: the correlation between weeks and months
If you’re at the beginning of your pregnancy and have convinced yourself that all you have to do to calculate how far along you are in months is to count from four weeks in four weeks, you’ll soon realise that the correlation between weeks and months is a little more complicated than that.
Here's how to calculate the exact month of your pregnancy:
First month: from the first day of your last menstrual cycle to 4 weeks + 3 days.
Second month: from 4 weeks + 4 days to 8 weeks + 5 days.
Third month: from 8 weeks + 6 days to 13 weeks + 1 day.
Fourth month: from 13 weeks + 2 days to 17 weeks + 4 days.
Fifth month: from 17 weeks + 5 days to 21 weeks + 6 days.
Sixth month: from 22 weeks + 0 days to 26 weeks + 2 days.
Seventh month: from 26 weeks + 3 days to 30 weeks + 4 days.
Eighth month: from 30 weeks + 5 days to 35 weeks + 0 days.
Ninth month: from 35 weeks + 1 day to 40 weeks + 0 days.
Pregnancy calculator chart
To make your life easier, we’ve summarised the information in a chart, which will help you understand how many months pregnant you are.
Save it to your mobile phone and use it whenever you’re not sure, so that the next time you’re asked, "how many months pregnant are you?" you’ll know exactly what to reply.