How to Calculate Due Date – Simple and Easy Ways

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. How do you calculate the due date? 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 (1). Let’s take a look at due date calculations in depth. Understanding these methods can help expectant parents better prepare for the arrival of their baby.

What Is the 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 (2).

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.

Why It Is Important to Calculate Due Date

Calculating the due date is important for several reasons, as it helps both healthcare providers and expectant parents prepare for a safe and healthy pregnancy and delivery. Here are the key reasons why determining the due date is crucial (3):

1. Monitoring Fetal Development: Knowing the due date allows healthcare providers to track the baby’s growth and development accurately. It ensures that the baby is meeting key milestones at the right stages of pregnancy.
2. Timing Prenatal Care: Prenatal checkups, tests, and screenings are scheduled based on the due date. These appointments are essential for monitoring the health of both the mother and the baby, identifying potential complications early, and ensuring timely interventions.
3. Preparing for Delivery: The due date helps parents and medical teams prepare for labor and delivery. It ensures that hospitals or birthing centers are ready, and parents can make necessary arrangements, such as packing a hospital bag or arranging childcare for older siblings.
4. Identifying Preterm or Postterm Risks: If a baby is born before 37 weeks (preterm) or after 42 weeks (post-term), there can be increased health risks. Calculating the due date helps identify these risks early, allowing for proper medical care and interventions.
5. Planning Maternity Leave: Expectant mothers can plan their work schedules and maternity leave more effectively when they know the approximate due date. This helps reduce stress and ensures a smoother transition during and after pregnancy.
6. Psychological Preparation: Knowing the due date helps parents mentally and emotionally prepare for the arrival of their baby. It provides a timeline for organizing the nursery, purchasing baby essentials, and planning for the new addition to the family.
7. Medical Interventions: In cases where medical interventions, such as inducing labor or scheduling a cesarean section, are necessary, the due date serves as a critical reference point for timing these procedures safely.

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 how to figure out the due date 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 (4).

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 14^th 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 (5). 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.

Other Ways to Calculate Your Pregnancy Due Date

Calculating your pregnancy due date is an essential step in preparing for the arrival of your baby. While the most common method is using the first day of your last menstrual period (LMP), there are other ways to estimate your due date, especially if your menstrual cycle is irregular or if you’re unsure about your LMP. Here are a few alternative methods to determine your due date:

1. Ultrasound Dating

An ultrasound, particularly during the first trimester, is one of the most accurate ways to calculate your due date. The sonographer measures the size of the fetus, specifically the crown-rump length (CRL), to estimate gestational age. This method is especially helpful if you’re unsure of your LMP or have irregular cycles (6).

2. Fundal Height Measurement

During prenatal visits, your healthcare provider may measure the fundal height (the distance from the pubic bone to the top of the uterus) to estimate gestational age (7). While this method is less precise than ultrasound or LMP, it can provide a general idea of your due date, especially in the second trimester.

How to Calculate the Due Date When You Had IVF

Calculating your due date after IVF differs from natural conception since the embryo transfer date is a key factor. Unlike traditional pregnancy calculations based on the last menstrual period, IVF due dates are more precise (8).

• For a Day 3 Embryo Transfer: Add 263 days (or 38 weeks) to your embryo transfer date to help in estimating your due date.
• For a Day 5 Embryo Transfer: Add 261 days (or 37 weeks and 2 days) to your embryo transfer date to determine your estimated due date.

Can the 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!

FAQs

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. 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.

4. What if you have long cycles or irregular periods?

A pregnancy wheel can help calculate the baby due date in such cases.

This was all about estimating or predicting the pregnancy due date. 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.

References/Resources:

1. Overdue babies; Better Health Channel; https://www.betterhealth.vic.gov.au/health/servicesandsupport/overdue-babies

2. Due Date Calculator; Cleveland Clinic; https://my.clevelandclinic.org/health/diagnostics/22052-due-date-calculator

3. Working out your due date; Pregnancy, Birth & Baby; https://www.pregnancybirthbaby.org.au/working-out-your-due-date#accurate

4. Lawson. G; Naegele’s rule and the length of pregnancy – A review (The Australian and New Zealand Journal of Obstetrics and Gynaecology); National Library of Medicine; https://pubmed.ncbi.nlm.nih.gov/33079400/; April 2021

5. Stokes. M, Olson. A, Chan’gombe. M, et al.; Reinventing the Pregnancy Wheel to Improve Pregnancy Dating and Antenatal Care Visits: A Pilot Randomized Trial in Malawi (International Journal of Maternal and Child Health and AIDS); National Library of Medicine; https://pmc.ncbi.nlm.nih.gov/articles/PMC8130211/; May 2021

6. Dating scan; Pregnancy, Birth & Baby; https://www.pregnancybirthbaby.org.au/dating-scan

7. Fundal Height; Cleveland Clinic; https://my.clevelandclinic.org/health/diagnostics/22294-fundal-height

8. Edwards. K, Itzhak. P; Estimated Date of Delivery; National Library of Medicine; https://www.ncbi.nlm.nih.gov/books/NBK536986/

Also Read:

How Accurate are Ultrasound to Predict Due Date
Reverse Due Date Calculator – How Does It Work?
What Is a Contraction Calculator and How Does It Work?
How To Calculate Pregnancy By Weeks, Months, & Trimesters?

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