Once you realise that you're pregnant the first burning question for both you and your doctor will be when you are due to deliver your baby. At your first doctor's appointment you will usually be given an 'estimated due date', although this may be adjusted later after a scan. Here's ThinkBaby's guide to due dates to demystify what they mean and how they're calculated.
What is a due date and how accurate is it?
Let's start by saying what a due date isn't. A due date isn't a certificate telling you the precise day your baby will be born, in fact, fewer than five per cent of babies are born on their estimated due date (EDD). The point of your due date is to give you an approximate idea of when your baby might arrive, and to give medical staff a yardstick against which to measure your baby's development in the womb. As your pregnancy develops, your doctor and midwife will use the due date to time your pregnancy scans, blood tests and other checks, as well as using it to check that your baby is growing within the normal range.
Although few babies are born on their due date, around 80% are born within two weeks of the due date, either before or after, so the due date does give you a good idea of the four weeks in which it's most likely that your baby will arrive.
Why does the due date matter?
A due date is useful to mums-to-be, giving them an idea of what to expect during the various different stages of pregnancy and helping them prepare for the arrival. Even more importantly, the due date helps medical professionals tailor and time your pre-natal care. Not only will the date be used to time tests and scans, but the size of your baby for its dates can have an impact on the prenatal care you receive. For example, a baby that is big for dates can be a warning sign for a maternal health problem such as gestational diabetes, while a baby that is small for dates may indicate an intrauterine growth restriction. Of course, being small or big for dates does not mean that there is necessarily a problem, perfectly healthy pregnancies can see bigger and smaller babies, but it will be something that health professionals check out to be sure that there's nothing wrong.
The due date is particularly important when it comes to birth. Your baby is deemed to be ready for birth at 37 weeks and if you go into labour before this point then your baby is said to be premature and will need extra medical care and attention both during and after labour. Your due date will also have an impact on how birth is handled if you go significantly over your due date, as women who are more than two weeks overdue may be advised to have the birth induced to speed the process up.
Calculating the due date - the basics
Most doctors calculate your due date using something called the pregnancy wheel, based on a regular 28-day cycle. This is the most elementary calculation for a due date. They will ask you what the date of the first day of your last period was, add a week to it and then subtract three months. For example, if the first day of your last menstrual period (LMP) was August 2nd, your due date would be May 9th.
Doctors also think of a pregnancy as lasting 40 weeks from first day of your last period, and that's also how most mums-to-be tick off the passing of time through pregnancy. Because doctors count this from the first day of your last period you may be surprised to be told that you're six weeks' pregnant when you are fairly sure that you only conceived about a month before, as you get two 'bonus weeks' before conception itself.
If you don't know when your last period was then you may be given an early dating scan to estimate when you are due.
Calculating the due date from ovulation/conception
Doctors use the wheel method because most women don't know when exactly they ovulated. However, as not all women have the text-book 28-day cycle with ovulation occurring around day 14, a more accurate due date could be given by dating the pregnancy from ovulation/conception and by adding 38 weeks to the ovulation date.
If you know that your cycles are shorter or longer than 28 days, your doctor may adjust your due date according to the length of your cycle, but will still assume that ovulation usually occurs about two weeks before your next period is due. For example, if you usually have a cycle of 35 days and the first date of your last period was on August 2nd, your doctor would estimate the date of ovulation/conception to be August 22nd, and the baby to be due 38 weeks later on the 15th of May, nearly a week after the date that you would be given by the standard calculation. You can read more about understanding your menstrual cycle here.
Most women should probably stop reading here and skip to the next section, but if you have irregular cycles or have been charting your ovulation times then read on for the really detailed part. Many women have irregular cycles and so will have no regular standard on which to base an approximate ovulation date. Even if you have regular cycles you don't necessarily ovulate 14 days before the end of your cycle: in a perfectly normal cycle a woman can ovulate between 12 and sixteen days before her period is due. What's more, even the most clockwork of cycles can be disrupted occasionally by factors such as stress. The only way you will know for sure when you ovulated is if you are charting your ovulation signs. If you are charting and know exactly when you ovulated then simply add 38 weeks to your ovulation date to arrive at your EDD.
In many cases the 'standard' due date and your own calculations may give a very similar due date, and there's no need for concern if your own calculations give you a date that's a few days off your official due date. If you try to suggest to your doctor that you take the date given by your own calculations as the official date then your suggestion will most likely be brushed off. If there is a significant discrepancy between your dates and the doctor's estimation then this is likely to be shown up when you have your dating scan towards the end of the first trimester, and the due date is likely to be adjusted to reflect the scan.
However, your own calculations may be more important if your pregnancy goes overdue and there is a significant difference between your calculation and the official dates. If your doctor thinks you are more than two weeks overdue s/he may well suggest induction, but if by your own (well documented) dates you're only a week overdue and your baby is in good health, you may have strong grounds to resist induction if you would prefer to wait.
But I didn't conceive then!
If you're not tracking your ovulation signs then you may be confused about the time your due date suggests you conceived, as it may suggest a date when you're sure you didn't have sex. In the first instance, don't be confused when the doctor tells you that you're x weeks' pregnant, and you know you didn't have sex x weeks earlier. Counting your pregnancy in weeks over 40 weeks always starts two weeks BEFORE conception itself, so at the time you conceive you are already technically two weeks' pregnant. this might sound confusing, but just be glad that you've already got the extra weeks under your pregnancy belt.
Even accounting for those two weeks the dates might still not stack up in your head, but it doesn't necessarily mean that the doctor's calculation is out. When it comes to conception what is most important is not the day you had sex, but the day you ovulated, which needn't necessarily be the same day. In the right conditions your partner's sperm can live in your uterus and fallopian tubes for up to five days, but they can't result in conception until your ovaries release an egg for them to meet. Once your ovaries had released an egg there's a maximum time-frame of 24 hours for conception to take place. So, it's perfectly possible that you may have had sex with your partner on Saturday and not have actually conceived until the following Wednesday when he was away on business, down the pub or standing in the supermarket queue.
Dating scans and adjusting the due date
Most women now receive a dating scan towards the end of the first trimester to check that the pregnancy is developing healthily and double-check the due date. The scan is usually done between weeks 10 and 14. Dating scans are usually very accurate in terms of judging how many weeks and days the fetus is at the time of the scan, and so can be used to provide an estimated due date. If there is a discrepancy between your given due date and the dating scan your due date may well be adjusted to reflect the scan.
While a dating scan or using your own ovulation dates may give you a more accurate idea of how far along in the pregnancy your baby is, and when your baby will be fully developed at 37 weeks, it won't necessarily give you a more accurate idea of when your baby will actually be born. When push comes to more pushing, it's ultimately your baby alone who decides when the time is right to leave the comfort of your womb.