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Congratulations on your pregnancy! Want to know when your baby is likely to be born? Use our due date calculator to estimate when you’ll finally get to meet your little one.
How do I use the HiPP due date calculator?
The HiPP due date calculator is a simple way to find out (roughly) when your baby will arrive. All you need to know is the date of the first day of your most recent period (your last menstrual period, or LMP) and the length of your cycle. Pop your dates into the tool below and press ‘calculate now’ to find out your approximate baby due date, and how many weeks pregnant you are.
If you can’t remember the exact dates, and your cycle is not regular, don’t panic. Babies don’t look at due date calculators – they come when they’re ready. This calculator is just intended to give you an idea of what you’re working towards. You will be given a more accurate estimate at your 12-week ultrasound.
When did I conceive based on my due date?
Many couples will speculate over which magical moment was the moment of conception. The truth is it can be hard to know. What we do know is that ovulation typically occurs about 10 – 16 days before your period arrives, which means you have a window during which conception is most likely This is usually about week three of your pregnancy, i.e. the third week after the first day of your LMP.
Conception doesn’t always occur on the day of intercourse and this can be confusing to some expectant parents. Sperm lives for up to a week, hovering around the fallopian tubes whilst an egg, once released, lives for around 12 hours. This means that effectively you could have intercourse and not actually conceive until a week later when the egg is released for the awaiting sperm to fertilise. Fascinating right?
Why is the due date calculated from LMP?
Partly because of the uncertainties around these dates, the date of conception is not usually used to calculate your due date. Using LMP gives medical professionals a more consistent framework. The other reason that LMP is used to calculate your pregnancy’s due date is because, with every menstrual cycle, your body is preparing for pregnancy.
When your due date is calculated from
Due date by last period
In most cases, LMP is used to calculate the expected due date – by adding 40 weeks (the average duration of pregnancy) or 280 days to the first day of your last menstrual period. This equates to approximately 9 months and 1 week. This is the measure healthcare professionals usually use to give you an ETA for your new baby.
If you don’t know when your last menstrual period was, don’t worry. Your healthcare professional will most likely refer you for an early ultrasound, which will enable them to estimate a due date based on the development of the embryo.
Due date from conception
If you’ve been actively trying for a baby, for example by using ovulation test kits, you might have more detailed insight on when your baby was conceived. While this is interesting, it won’t usually affect how your midwife estimates your due date – they will still use the LMP method until you have an ultrasound to give more precise results.
IVF due date
If you’ve undergone IVF, your due date is estimated based on a combination of the date and type of transfer. Because transfer will occur at the most fertile point in your cycle, the due date is calculated at 38 weeks from the point of transfer, less the number of days the embryo is developed (e.g. 38 weeks minus 3 days, or 38 weeks minus 5 days).
Frequently asked questions about due date calculators
Are due dates usually accurate?
The short answer to this is no. Due dates give you an idea of when your baby will reach 40 weeks’ gestation; not when they will decide to make an appearance. It’s estimated that less than 5% of babies arrive on their due date. Pregnancy is usually considered to last between 37 and 42 weeks – quite a range! – and your baby’s arrival could fall some way from their due date. For this reason, many expectant parents find it useful to think in terms of rough weeks or even months, rather than specific days.
Why is a due date important?
Calculating your due date enables you to chart your baby’s development. For health professionals, the due date (officially calculated during the 12-week ultrasound scan) provides the basis by which they measure growth. If the baby is not growing as expected, your midwife or doctor will be able to identify if there are any issues through additional monitoring. This might result in extra growth scans, which include checking all is well with the blood flow from the placenta to the baby. In the later stages of your pregnancy, these checks will help to establish whether intervention is recommended – e.g. to induce labour early. It’s worth noting that although babies' growth is measured based on national averages, many babies are just born little, and are perfectly healthy! This is often the case with parents who themselves were smaller than average at birth.
From a personal perspective, it’s nice to monitor your baby’s progression through the pregnancy by following along with a week-by-week pregnancy diary, for example, which tells you what you might experience at any given point in your pregnancy, and what’s going on with your baby. This not only gives you some ideas of what to be mindful of – e.g. diet, baby movements, etc. – but also helps with bonding as you ‘get to know’ your baby. And, of course, knowing when your baby is likely to arrive, also helps with all the practical preparations, like packing your hospital bag, writing a birth plan, and preparing your home.
Can my due date change?
Yes, your due date might change between your first booking-in appointment – when you’re working on the basis of dates you’ve given your midwife (the same dates as you put into this due date calculator) – and your 12-week ultrasound scan. This is because the ultrasound scan provides a more accurate picture of your baby’s development. When you give your LMP to your midwife, they work out your due date based on that, assuming you have ovulated in the middle of your cycle. Not everyone does, however. Some will ovulate early in the cycle, and some later. Having the ultrasound at 12 weeks is more accurate as the baby changes vastly from day-to-day and week-to-week in the first trimester. Your sonographer will undertake many measurements during this and the 20-week scan that help give a better timeline.
Can I plan my due date?
If you’re actively trying to get pregnant, you can of course decide whether you want to try for a spring baby, a summer baby, etc. However, these things don’t always pan out as expected and some couples can be trying for a long time before they fall pregnant. Even if you do conceive as planned, your baby’s arrival is not guaranteed to fall exactly at 40 weeks. They come when they come. Getting your head around this is good practice for being new parents, who are also at the mercy of their little one’s schedule!
Marley Hall is an award-winning midwife, hypnobirthing instructor, mother of five and author of 'Midwife Marley's Guide For Everyone: Pregnancy, Birth & The Fourth Trimester'. For more information, see here.