How to Calculate Pregnancy Due Date
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An average pregnancy lasts 40 weeks while the actual gestation period in humans is about 38 weeks. This is why a baby born any time after 37 weeks is considered a full-term baby. The due date is usually calculated by one of the different methods such as LMP, Naegele’s rule, or the pregnancy wheel. All these methods help arrive at an estimated date and only about 5% of women actually give birth on their due date. Let’s take a look at due date calculations in depth.
What Is Pregnancy Due Date?
The date when your child is expected to be born is known as the pregnancy due date. This is usually about 40 weeks from the first day of your last menstrual period. But if you know the exact date of conception then your due date will be 38 weeks from that day.
The due date might be revised by your doctor during the course of your pregnancy depending on the growth of the child or for other reasons. A due date helps your doctor decide when to prescribe various prenatal tests, estimate the growth of the foetus, and take a call on whether any intervention is required if you are overdue.
Calculating Pregnancy Due Date
Every doctor has his or her own preferred method of arriving at the pregnancy due date. You may come across a variety of due date calculators and tools online. However, the three most popular methods in use are:
1. Using Naegele’s rule for calculation of pregnancy due date
Named after German obstetrician Franz Karl Naegele, this rule assumes that a pregnancy lasts for 280 days and one menstrual cycle is of 28 days duration. This means that your expected due date (EDD) is arrived at by adding 280 days (9 months and 7 days) to the first day of your last menstrual period. But if you have a shorter menstrual cycle of say 27 days, you will need to add one day less, i.e. 9 months and 6 days, and so on. If you have a longer cycle, then you have to add the extra days to 9 months and 7 days to arrive at your EDD.
2. Conception date method to calculate your pregnancy due date
Also known as the LMP (last menstrual period) method, it tries to figure out the date on which you conceived to arrive at your EDD. This works on the theory that most women have a 28-day menstrual cycle and in this case, ovulation occurs on or around the 14th day. In this method, forty weeks are added to the date on which you got your last menstrual period. From this date, you take away two weeks to get your EDD. This is to allow for ovulation which is assumed to have happened within the first two weeks after your last period.
3. Pregnancy wheel
Also called a gestation calculator, a pregnancy wheel has become an easy way to estimate your EDD. It does the calculation based on the date of your last period.
Usually, it also contains information about the different stages of your pregnancy. A pregnancy wheel has different spokes displaying the months in a year, phases of antenatal development, as well as start and end of the trimesters.
Can Calculated Delivery Due Date Be Wrong?
None of the EDD calculation methods is foolproof and cannot guarantee that your child will be born on a particular date. It is normal to give birth before or after your estimated due date up to two weeks either way. Statistics indicate that only about 1 in 20 women deliver on their due date. About 10 per cent of first pregnancies have been found to continue for two weeks after their EDD. Also, around 70 per cent of pregnancies that go overdue are usually wrongly dated.
Pregnancy Due Date Tips
As your due date approaches, you are sure to go through a range of emotions – from anxiety to joy to relief that the long wait is almost over. Your physical discomfort is sure to be at its peak during this time due to the baby running out of space inside. If it’s your first baby, then the prospect of labour and delivery might get you jittery. So, here’s what you can do in those last few days:
Exercise – Continue with whatever exercise you have been pursuing throughout your pregnancy. It can help induce labour and keep you supple for delivery.
Double Check Hospital Bag – It doesn’t hurt to take one more look at what you have packed in your hospital bag. See if there is anything you might have forgotten that you or baby might need for the first couple of days.
Sleep, Sleep – Catch up on sleep as much as you can with naps during the day too. Once your baby is home, sleep might become a scarcity, especially during the first few months!
1. How accurate are these pregnancy due dates?
Around 5% of all babies are born on their due date of delivery. A whopping 90% of babies are born within two weeks of either side of the estimated due date. Any time between 37 weeks to 42 weeks is considered normal.
2. Can the due date change?
Yes, the due date can change depending on different factors like the development of the foetus, pregnancy complications and overall health of the mother.
3. Are there any other methods to calculate the due date?
Yes. Ultrasounds can also help you arrive at an expected due date. These are usually performed sometime between eight and twenty weeks of pregnancy. Measurements of the various parts of the baby’s body and organs helps doctors and radiologists arrive at an estimated gestational age. First trimester ultrasounds are thought to be the most accurate in due date estimation.
4. What if I don’t know my last LMP (last menstrual period) date?
There are other ways in which your doctor can determine the due date, such as an ultrasound.
5. What if you have long cycles or irregular periods?
A pregnancy wheel can help calculate the baby due date in such cases.
Besides the obvious benefit of knowing when your child is likely to be born, knowing when you are due can prove helpful in some other ways. It gives you an idea of the time you have in hand to get the nursery ready and also helps determine when to start your pregnancy and maternity leave. But remember that this date is just an approximation and not something to get stressed out about. Just stay focused on your health and the well-being of your unborn child as you count off the days!
Disclaimer: This information is not intended as a substitute for medical advice from a qualified professional.