Home » Baby Due Date » How To Calculate The Baby’s Due Date? Let Us Help You With It

How To Calculate The Baby’s Due Date? Let Us Help You With It

How To Calculate The Baby’s Due Date? Let Us Help You With It

What is the due date?

A baby develops in your womb safe from the outside world. However, he has to come out at some point and take care of himself. The day he comes out is the due date also known as the estimated date of delivery (EDD). There are many ways to calculate the baby’s due date, however, you should be clear that this date is just an estimate. Still, most of the pregnant ladies deliver within a week of this date on either side. So it is important to know the baby’s due date to get all the preparations done.

Gestational versus Fetal age

Gestational age begins from the day the woman conceives the baby to the day the baby is born. So you calculate it from the first day of the last menstrual period. However, the fetal age begins when the fetus gets implanted into the uterus. So the fetal age is the actual age of the baby and gestational age is the duration of the pregnancy.

Since you don’t know the day of implantation of the embryo, gestational age is easier to use. A normal gestational age is anywhere between 38 to 42 weeks. The baby born before 37 weeks is known as premature.

How to calculate the baby’s due date?

There are several methods to calculate the baby’s due date with different accuracies. However, the easiest of the method which even a layman can use is by LMP. All you need is to add 280 days to the first day of your last menstrual cycle. However, you have to know the first day of your LMP and your cycle has to be regular i.e. 28 days. So it is better to keep the track of your menstrual cycle.

You need to know that even after knowing all this it is just an estimate and it can be wrong.

There are several methods to calculate the baby’s due date using LMP:

·        Baby’s due date from pregnancy wheel calculators

Different pregnancy wheel calculators are available which can help you estimate the baby’s due date easily. These calculators use Naegele’s rule to calculate the due date.

In this rule, you simply add 280 days to the first day of your last menstrual period. However, for this, your menstrual cycle has to be regular. But if you have the calculator, it does it for you. Besides, the due date it provides information about other significant milestones of the pregnancy as well.

·        Mittendorf-Williams rule

It is an advanced rule with greater accuracy because it includes more details. This rule is based on the fact that the mothers having first pregnancy keep their baby for 288 days. This makes gestation last for an extra 8 days. However, this rule adds an extra 3 days to the 2nd or later pregnancies.

To calculate the baby’s due date:

First pregnancy: Add 3 months and 15 days to the LMP.

2nd or later pregnancy: Add 3 months and 10 days to the LMP

·        Parikh’s rule for baby’s due date

If you are having a regular menstrual cycle you will feel less difficulty calculating your due date. However, if you are having irregular cycles things are a bit difficult.

The rule says to add 9 months to the first day of the last menstrual period the subtract 21 days and add the days of your last menstrual period. This is a modified rule with an aim to reduce the error caused by an irregular cycle.

·        Wood’s Rule for baby’s due date

It is more detailed which takes into account the variability of the menstrual cycle as well as the number of pregnancies. For this, you have to calculate the estimated date of delivery and then correct it according to your menstrual cycle. It makes the baby’s due date even more accurate.

To calculate EDD:

For the first pregnancy: 1st day of LMP + 1 year – 2 months and 2 weeks.

For 2nd or later pregnancy: 1st day of LMP + 1 year – 2 months and 2.5 weeks.

Then to calculate baby’s due date:

For cycle > 28 days: EDD + ( actual days of menstrual cycle – 28)

For cycle < 28 days: EDD – ( 28 – actual days of the menstrual cycle).

However, if you don’t have an idea of your last menstrual cycle or the day of your conception things can get a little tricky. In that case, you need to visit your doctor to get the baby’s due date calculated. Your doctor can use the following methods to calculate the baby’s due date:

  • Belly height

Not a rule but most of the time the height of your belly bump in centimeters corresponds with the weeks of your pregnancy. Calculating the current week of pregnancy you can have an estimate about the baby’s due date by adding the weeks required to make it 40 weeks pregnancy.

It is a very crude method and requires professional help and shouldn’t be solely relied upon.

  • Ultrasound

A more accurate way to calculate the baby’s due date when you don’t know your LMP is by ultrasound. Your doctor will work out the current age of the fetus through his developmental landmarks. By making the weeks to a total of 40 weeks, you can get a relatively accurate calculation of your baby’s due date.

What to do with the baby’s due date?

Here are a few benefits of knowing your baby’s due date beforehand:

  • You can plan the day of delivery beforehand and make the required arrangements instead of panicking.
  • Knowing the baby’s due date can help you keep the record of your baby’s growth. You can closely observe the developmental milestones of your baby.
  • It helps you plan the hospital visits, scans, and blood tests in advance.
  • You can plan a dream house or a dream room for your baby in advance.

Knowing the baby’s due date is pretty simple if you know the first day of your last LMP. It helps you plan accordingly and makes you a responsible mother!

Pregnancy Calculator

Conception date
Date of 3-day embryo transfer
Date of 5-day embryo transfer
Due date by sonogram
First day of last menstrual period

Frequently Asked Questions

What percentage of babies are born exactly on the predicted due date?

You should know how to calculate the baby’s due date, but you must also know that it is just an estimate. You cannot expect your baby to be born exactly on the expected date of delivery. According to statistics, only 4% to 7% of babies are born on the calculated due date. You should be mentally prepared to deliver your baby from 3 weeks before the due date to 2 weeks after it. So basically, you get an accurate idea about the due month but not the due date. It is useful to be aware of the following terms,

  • Preterm: before 37 weeks
  • Early term: between 37 and 38 weeks
  • Full term: between 39 weeks and 40 weeks
  • Late-term: 41st week
  • Post-term: after 42 weeks

A 2013 study analyzed the data of 18,708 women in Australia with spontaneous labor and compared the estimated date of birth (eDOB) with the actual date of birth (aDOB). It concluded that only 5% of births occurred on the eDOB. However, approximately 66% occurred within 7 days of the eDOB.

According to the Centers for Disease Control and Prevention (CDC), 10.02% of babies were born preterm in 2018 in the United States. 26.53% were born in weeks 37-38; 57.24% in weeks 39-40; 6% in week 41; and less than 1 % at 42nd weeks or beyond.

Is it more likely for a baby to be born before the calculated due date or after it?

You can estimate it from your previous pregnancies. If your last baby was born before the due date, you are likely to have this baby before the due date as well. If you are a first-time mom, you should know that your genetics play a role too. Ask your mother if her pregnancies were preterm, early term, full-term, or late-term. If your mother had her deliveries after the due date, you are likely to have your baby after the calculated due date as well.

According to the data on the distribution of births by gestational age from the Centers for Disease Control and Prevention (CDC), the number of babies born before the calculated due dates is increasing and post-term pregnancies are decreasing with time. This may be attributed to an increased number of C-sections and induction methods.

The following chart shows the comparative data between 1990 and 2005 on the distribution of births by gestational age.

How do doctors calculate the baby’s due date?

The most commonly used method is based on your last menstrual period (LMP). You can calculate the baby’s due date by simply adding 280 days to the date of your last menstrual period. However, you must be precise about your last menstrual period to calculate the baby’s due date accurately.

A more accurate way to calculate the baby’s due date is an ultrasound. Your doctor will calculate the current age of the fetus from the developmental landmarks. This is particularly more useful in early pregnancy when you can get a good estimate of how old the pregnancy is from the size of the fetus.

What is more accurate: ultrasound due date or LMP?

According to the American College of Obstetricians and Gynecologists, fetal measurements taken during a first-trimester ultrasound are the most accurate way to estimate the due date. Moreover, research by P. Taipale and V. Hiilesmaa compared the Ultrasound and LMP methods to calculate the baby’s due date and found ultrasound more accurate. The number of post-term pregnancies decreased when it was used.

Well, the main reason that the last menstrual period method is not very accurate is that most women do not know the date of their last menstrual period accurately.

  • Many women do not keep track of their cycles or sexual intercourse.
  • They may have irregular cycles because of taking a contraceptive pill or conditions such as Polycystic Ovarian Syndrome (PCOS).

They may have confused implantation spotting with periods. Implantation spotting is the bleeding that occurs in some women when the baby implants in the uterus.

How to calculate the baby’s due date if I have PCOS?

PCOS or Polycystic Ovarian Syndrome is a condition that affects a woman’s hormone levels and causes infrequent or prolonged menstruation. The length of the cycle is usually more than 35 days. Hence the LMP method may not be useful to calculate the baby’s due date.

However, even if you have PCOS, it is still possible for you to determine the date of ovulation using ovulatory tests based on the levels of Luteinizing Hormones (LH); changes in your basal temperature and cervical mucus. You simply have to add 266 days to the date of ovulation to calculate the baby’s due date.

Even if you were not able to track your ovulation day, ultrasound exams based on the size of your fetus are pretty accurate to calculate the baby’s due date.

What is a pregnancy due date calculator?

A pregnancy wheel calculator helps you to calculate the baby’s due date easily. It requires you to enter the first day of your last period and the length of your cycle. And it uses this information to calculate your due date.

These calculators use the Naegele’s rule to calculate the baby’s due date, which involves adding 280 days to the first date of our last period if you have a 28-day cycle.

Is it possible for a woman to refuse to give birth and hold the baby past its due date?

The due date is an estimated date of pregnancy. You can have your baby after the due date but you cannot “hold the baby in” after labor starts. There is a rush of hormones that increase the contractions of your uterus. Your cervix dilates. Your water breaks. It is an involuntary process. So you cannot make your uterus stop contracting or prevent your cervix from dilating.

If you intend to hold the baby post its due date by not pushing or trying to keep it inside, you really shouldn’t. Your baby can go into distress and defecate in the womb.

Is it dangerous if the baby passes the due delivery date and is not born yet? And what is the longest the doctor will allow a pregnancy to go past the mother’s due date?

Your due date is just an estimated date of delivery. Your baby may be born two weeks after the estimated date. However, an overdue pregnancy can cause serious complications. It increases the chances of stillbirth. The placenta might not be able to do its job correctly and infections may develop inside the womb. The risk of breathing problems and C section increases.

“How long your doctor can wait for your baby to be born after the due date” depends on:

  • How much overdue is the baby
  • What is the age of the mother
  • Whether she has given birth before
  • Does she smoke
  • What is the weight of the mother
  • How big the child is
  • Are there any signs of danger

A doctor may wait even 3-4 weeks after the due date if there are no complications. Albeit, to decrease the risk of complications, doctors induce labor in the first week after the due date by giving hormones that cause your uterus to contract artificially.

Leave a Reply

Your email address will not be published. Required fields are marked *