It can be tricky to work out your pregnancy due date, this is because of a number of factors, including the length of your menstrual cycles and even your family history. If you’re having trouble remembering dates and counting up the days on your fingers and toes, don’t worry – use our pregnancy due date calculator to work out how many weeks pregnant you are.
Due date calculators are clever little tools that work out your due date from the date of your last period. However, it is important to remember that due dates are an estimate and can vary. The amount of babies born on their actual due date is less common than you think, with many mums giving birth weeks and days before and after as babies have a habit of not being very good at being on time. With expert help, we have answered loads of FAQs about due dates below, including whether you can rely on due dates and how likely it is that your baby will be born on their due date.
How to use the pregnancy calculator:
To use Mother&Baby's due date calculator you simply need to work out the first day of your last period and roughly how many days your menstrual cycle is (the average cycle is 28 days). The tool will calculate 40 weeks from the first day of your last menstrual period, even though your baby is technically about two weeks younger than that. If you are unsure as to whether you had a full period or an implantation bleed, check out our guide here on the difference betweeen the two.
How accurate is a due date calculator?
Unless you can pinpoint exactly what point of your ovulation cycle you were in at the time of fertilisation, it’s difficult to know what date exactly you’ll meet your little one. However, the pregnancy calculator can give you a good estimate. There are also conception calculators where you can find out a rough estimate of when you conceived your little one.
Only about 4% of babies are 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. While this tool can give you a rough guide of when to expect your baby, we know all too well that baby will arrive when they are good and ready.
Alternative ways to working out your due date
Most women ovulate and conceive two weeks after their last period (or LMP for Last Menstrual Period in fertility speak) and will only be around 38 gestational weeks by the time the pregnancy has reached the 40-week mark.
"Your dating scan, which you’ll have at 10 to 14 weeks is the most accurate scan to determine your due date," explains expert Teresa Walsh, a midwife at The Portland Hospital, London.
To work out the length of your menstrual cycle, count from the day your period normally starts to the day before your next period. The average cycle is 28 days.
Ready to brush off your GCSE maths? Take the first date of your last normal period, add seven days to that number. Take that date, add nine months, and you get your due month. Simple! Although, don’t be surprised if you add back and find you didn’t have sex on that exact date.
All babies grow at an individual rate and your due date is only an estimation, so be as prepared as you can be for a slightly-earlier-than-expected surprise, or carrying your baby for a little longer than that date you now have fixed in your head.
Keep in mind that women with [irregular cycles]{href='https://www.nhs.uk/conditions/irregular-periods/' }may have difficulty calculating their due dates (the LMP method is far from a perfect science), so if you fall in that camp talk to your doctor to get the best estimate of your due date.
Babies born via IVF will have their due date calculated from date of embryo implantation, not LMP or ultrasound.
How likely is it my baby will be born on their due date?
As we mentioned above, only 4% of babies are born on their due date. Your due date is calculated from the end of your last period, plus 40 weeks or 280 days. That said, it is very likely your baby will arrive a couple of weeks either side of your due date, so it's a good idea to keep this date in mind and get prepared.
How do I work out how many weeks pregnant I am?
Pregnancy normally lasts from 38 to 42 weeks. It can be tricky at first to work out how to count your pregnancy in weeks and months, as when we measure your pregnancy from the start of your last period, your baby is actually two weeks younger than the pregnancy start date, otherwise referred to as the 'gestational age'. So, this means when your doctor tells you you're four weeks pregnant, your baby's gestational age is actually two weeks.
What is ‘full-term’ pregnancy?
It’s not as simple as assuming your full-term pregnancy is the day you hit 40 weeks pregnant. In fact, your full-term pregnancy is considered to be anywhere from 38 to 42 weeks. Yes, it’s a due month.
"On average pregnancy takes 40 weeks (or 10 lunar calendar months) from the first day of your menstrual cycle or 38 weeks from conception, but there isn’t an exact science to working out your due date," says Teresa.
What is a dating scan?
During your first ultrasound scan, also know as your dating scan, your midwife will be able to give you a more accurate guide to your due date. You will normally have a dating scan between 10-14 weeks pregnant, so it is often referred to as the 12-week scan. At this time, the sonographer will look at how developed your baby is and measure the length of your baby from his head to his bottom, which helps them to give you an EDD (estimated due date) based on how far along in your pregnancy you are.
They will also check whether you are expecting one baby, or more, that your baby is developing in the right place, and can screen for early signs of some abnormalities using a nuchal scan.
What is a trimester?
Trimester means 'three months', so your pregnancy will be divided into three trimesters. Your first trimesterwill be from conception to thirteen weeks and six days. Your second trimester is from 14 weeks to 27 weeks and six days. Your third trimester is from 28 weeks to when you give birth.
Can my due date change?
Yes! Your due date can change due to a multitude of reasons, including incorrect calculations in the early weeks, or baby developments suggest your baby is further along (or less developed) in later scans, including your levels of alpha-fetoprotein (AFP), a protein made by the baby, being outside the usual range. If you have any questions or worries talk to your doctor.
Can I plan my due date?
Whether you are hankering for a summer baby or fancy not giving birth on Christmas day, you may be looking to 'plan your due date'. It's verging on impossible to plan your due date exactly, even if you are lucky enough to fall pregnant when you want to. As echoed throughout the article, only 4% of babies are born on their due date, so even if you have fallen pregnant in your chosen month, it's hard to determine which day/week your little one will arrive.
What to do after you have calculated your due date
Choose a hospital or maternity care provider
You will be seeing a lot of your practitioner over the next 9 months, so have a look around at who would be a good match. Antenatal care providers often schedule a meeting for when you are 8 weeks pregnant.
Confirm you're pregnant
If you've only gone by early pregnancy symptomsor a gut-feeling you're pregnant, take a testor visit a doctor to confirm.
Start taking vitamins
You don't need to wait until you've seen a doctor or midwife to start taking folic acid and vitamin D supplements, both of which are essential for a healthy pregnancy.
Stop drinking alchohol and smoking
As soon as you know you're expecting, you should give up cocktails for the forseable.
Join a Facebook group
Be sure to join our Facebook group #mumtribe, which is our fast-growing community of mums asking for advice, sharing challenges and small wins (like getting to the bottom of a hot cup of tea) and reminding each other, no one is perfect!
Start thinking about names
Check out our baby names generator, which is full of wonderful name inspiration to get you excited for your new arrival
Use our [Chinese Gender Predictor]{href='https://www.motherandbaby.com/pregnancy-and-birth/pregnancy/chinese-gender-predictor-tool' }
This chart, found in an ancient tomb near Beijing more than 700 years ago, is said to be 90% accurate in predicting your baby's gender (which is better than the 50/50 odds of how high your bump is, right?)
Get clued up on maternity leave
Whether you’re a self-employed mum-to-beor planning on returning to work once your baby’s born, it’s sensible to know exactly where you stand on maternity pay and benefits.
Enjoy alone time with your partner (if you have one)
If you want to keep schtum about your news until the 12-week scan, you and your partner can still have fun with the we-know-something-you-all-don’t-know excitement.
Hannah Mellin is Digital Writer at Mother & Baby. Hannah is a mama to a one year old little boy and is is navigating her way through first-time parenthood, every developmental leap at a time. She is up for trying anything and everything to make mums feel less anxious and overwhelmed.