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Due Date Calculator

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length of your cycle: 28 days
your cycle is the number of days between starting one period and starting the next

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* Legal disclaimer: This result is only an estimate. Baby will come when its ready. Consult your healthcare provider to determine your final due date.


Congratulations on the beginning of your new adventure!


There’s probably a lot on your mind at the moment, from learning about breastfeeding and sleep safety to planning what the nursery will look like.


But first, your pregnancy journey – time flies and before you know it, you’ll have a baby in your arms.


Pregnancy (or gestation) usually lasts between 37-42 weeks from the first day of your last period, which means its length can vary naturally by as much as five weeks.[1]


If you give birth before 37 weeks, your baby in considered premature and may need extra care.


If you give birth after 42 weeks, without any signs of impeding labour, your pregnancy is deemed prolonged or overdue.


Keeping track of all the dates, possibilities and how many weeks pregnant you are can be tricky – this is where a due-date calculator can help.


due date calculator pregnancy

How Does the Due-Date Calculator Work?


A due-date calculator, or pregnancy calculator, is a simple, useful tool that uses mathematics to give you an estimated date of delivery (EDD).


It works out the date your baby is due to arrive by adding 40 weeks (280 days) to the first day of your last menstruation (not from the first day of a missed period), considering a cycle length of 28 days. This can be adjusted to your own cycle length, whether shorter or longer.


The calculator can also work out your due date and how far in your pregnancy you are by adding 38 (266 days) to the day of conception; this day is, however, very complicated to determine – the moment of conception is almost never known.[2]


This is because it’s difficult to pinpoint your ovulation day, and with sperm being able to survive up to five days and the egg twenty-four hours, it can happen that conception occurs several days after you’ve had unprotected sex.


Although it may sound strange, your period and ovulation are counted as the first two official weeks of pregnancy.


This means that during these initial weeks, you aren’t technically pregnant – your body is simply getting ready for ovulation[3], and consequently pregnancy.


Lots of changes happen during this time which are considered vital for a healthy pregnancy.[4]



How Accurate Is the Due-date Calculator?


The due-date calculator only estimates your baby’s date of birth. With so many factors at play, it’s impossible to predict with precision when your baby will arrive.


As previously mentioned, pregnancy length can normally vary by as much as five weeks.


The calculator is useful because it allows you to plan for the time of year when baby is due to make an appearance, even if you can’t write down the exact date of the big day in your diary.


You can get a more accurate idea of when your little one is due to arrive by getting a dating scan (ultrasound) as a part of your antenatal (pregnancy) care.


Your midwife will then be able to tell you about your pregnancy and baby's development and how far along you are in your pregnancy with more precision.



Can You Plan Your Due Date?


It’s not possible to choose the exact date to welcome a new arrival into the world.


However, it’s possible to try and time when you conceive to “plan” your estimated time of delivery.


Keep in mind that although you might be lucky enough to get pregnant when you intend to, it’s unlikely that you'll be able to determine when you’ll give birth with precision to the day, or even week or month.[5]


Only about 5% of babies are born on their due date.[6] It’s perhaps easier to think and prepare for your little one to be a winter, spring, summer or autumn baby.


due date calculator pregnancy

Can Your Due Date Change?


Your due date is not fixed. Several factors can affect your estimated date of delivery and lead to your doctor revising it.


This doesn’t tend to happen a lot and depends on how your due date estimated in the first place.[7]


A possible reason behind a change in your due date may be irregular periods, as standard calculations are based on a regular 28-day cycle.



What Are the Different Ways to Calculate Your Due Date?


Naegele's Rule


A three-step method, Naegele’s Rule determines your estimated time of delivery by counting back three months from the first day of your last period.


You then add one year and seven days to that date. This approach is based on a 28-day menstrual cycle.[8]


For example, if your last period started on the 15th of September 2019. Counting back three months takes you to 15th of June 2019. Then add one year and seven days, the estimated time of delivery is the 22nd of June 2020.


Mittendorf-Williams Rule


More complex than the Naegele's Rule, the Mittendorf-Williams Rule takes more factors into consideration to estimate your due date such as age, consumption of coffee, and weight, among others.[2]


This approach is based on a study which demonstrated that first-time pregnancies tend to be slightly longer than those of women who have previously been pregnant.


Parikh's Rule


Named after a German obstetrician, Parikh’s method was developed upon the discovery of the inaccuracy of Naegele Rule in estimating the due date for women with a cycle longer or shorter than 28 days.


It works out the date your baby is predicted to arrive by adding nine months to the first day of your last menstrual period, followed by subtracting twenty-one days and adding the number of days in your average cycle.[9]


Wood's Rule


Also called Nichols’ Rule, Wood’s method estimates your due date by adding twelve months to the first day of your last period, followed by subtracting two months and 14 days, which amounts to a pregnancy duration of 41 weeks.


This formula is designed for women whose menstrual cycles last 28 days but can be adjusted for shorter or longer cycles.[9]


Due-date Calculators


Different due-date calculators may use different approaches in estimating the day your baby will arrive.


Our calculator uses Naegele's algorithm as the main rule to determine your estimated date of delivery, with the possibility to adjust according to the length of your menstrual cycle.


due date calculator pregnancy

Preparing for Your Due Date


Once you know your baby’s estimated date of arrival, you can start planning for the big day.


Don’t leave everything to the last minute, as your little one may decide to make an early appearance.


If you’re in your third trimester, it’s a good idea to start thinking about what to pack in your hospital bag, as well as stocking up on baby essentials.



Read More:


  • How to Have a Healthy Pregnancy

  • Pregnancy Mood Swings

  • The Signs and Stages of Labour


  • Approved by Mothercare Parenting and Medical Experts



    Last reviewed: September 2019



    References:


    [1]. Due Date Calculator, NHS, April 2018

    [2]. Due Date Calculator, Flo Health, [Accessed September 2019]

    [3]. Pregnancy at 1 to 3 Weeks, NHS, July 2019

    [4]. 2 Weeks Pregnant, Flo health, [Accessed September 2019]

    [5]. Due Date Calculator, What to Expect, [Accessed September 2019]

    [6]. Due Date Calculator, Emma's Diary, [Accessed September 2019]

    [7]. Why Due Dates Changed, The Bump, March 2019

    [8]. Calculating a Due Date, Hopkins Medicine, [Accessed September 2019]

    [9]. Which Pregnancy Calculator Should You Use, Lifestyle Medical Centre, May 2016