How many weeks pregnant am I?
To work out how many weeks pregnant you are, and your due date, you will need to know the first day of your last menstrual period (LMP) – when your last period began. Simply enter this information into our calculator and click the ‘Calculate’ button. The pregnancy calculator will also estimate the conception date.
How do I calculate how many weeks pregnant I am?
Your due date isn’t calculated from the day you conceive – even if you know when you ovulate. There is no way to know that the sperm fertilised the egg on that same day – an egg can be fertilised for 24 hours after ovulation, and sperm can survive for three days after intercourse.
Your due date is calculated based on the date your last period started, and assumes you have a regular cycle. The first day of your last period is considered the first day of your pregnancy. 40 weeks is then added to this to give you your estimated due date, and an indication of how many weeks pregnant you are.
Find out what happens week-by-week during pregnancy.
Why is my due date calculated by the first day of my last period?
It is standard practice for doctors to estimate due dates from the first day of your last menstrual period. To calculate your due date you need to know the date of your last period, the length of your cycle and that your cycle is regular.
If you don’t know the length of your cycle you can leave it as the default of 28 days, which is considered a standard cycle.
You may be interested to know that the menstrual and ovulation period count as the first two weeks of pregnancy, even though you aren’t actually pregnant!
**Due dates can be revised based on the size of your baby during the viability scan (also known as a dating scan)**
How accurate is the pregnancy calculator?
Our calculator uses the same method that doctors and obstetricians use for estimating how many weeks pregnant you are and your due date.
Any estimation of due date is always just that – an estimate – data from the Perinatal Institute says that just 4% of babies are delivered on their due date.
What trimester am I in?
Once you’ve discovered how many weeks pregnant you are, you’ll likely be asking ‘Which trimester am I in?’ Your can find out which trimester you are in by comparing how many weeks pregnant you are to our guide, below:
First trimester: 1-12 weeks
Second trimester: 13-26 weeks
Third trimester: 27-40 weeks (or when you give birth to your little one)
What can I expect during my pregnancy?
Read the NHS guide to what to expect from your pregnancy week-by-week for more information on what to expect throughout your pregnancy, including the physical and emotional changes you’re likely to experience.