Due Date Calculator – Know When Your Baby is Due

Pregnancy is an exciting time in the life of every woman. As soon as your pregnancy test turns positive, you want to know everything you can about pregnancy. There are thousands of questions in your mind as you start preparing for this incredible journey. One of the first questions that will pop into your mind is – when is my due date?

You can use the above due date calculator to calculate your due date. To calculate your estimated due date, you need to be aware of the first day of your last period or the date of when you conceived. Women are usually not aware of their pregnancy until they miss their period. By that time, you may already be five weeks pregnant.


Different ways to calculate due date

The estimated due date is the date when the spontaneous onset of labor is expected. You can choose from a variety of ways to determine your estimated due date (EDD).

1. First day of your last period

Most pregnancies last 40 weeks. You can easily calculate your due date by counting 40 weeks (280 days) from the first day of your last menstrual period. The pregnancy wheels used by many obstetricians and midwives work on this method.

You can also use Naegele’s rule to calculate your due date quickly. To use this method, take the first day of your last period and add one year to it. Subtract three months from this date and add one week. So, if your last period started on March 11, 2020, you move the date forward by one year, i.e., March 11, 2021. Now count back three months to December 11 and add seven days. This means your due date is December 18, 2021. However, you must remember you can deliver up to two weeks before or after.

2. Conception or ovulation date

Since the human egg is capable of fertilization for only 12 to 24 hours after ovulation, we can take the date of ovulation as the date of conception. To estimate the estimated due date from your date of conception, just add 266 days to the date of conception.

Doctors consider your due date based ovulation more accurate than the one calculated based on your last menstrual period. Share the information about the length of your ovulation cycle and the date of your ovulation with your doctor.

3. IVF transfer date

According to the CDC, there are more than 250,000 assisted reproductive technology cycles performed in the US every year, resulting in around 77,000 babies being born. If you have had IVF treatment, your doctor can calculate your due date precisely by using your IVF transfer date. Since the fertilization and implantation are observed events during IVF treatment, your doctor can make accurate calculations of your due date.  For example, if you had a Day 5 embryo transfer, your due date is 261 days later

4. Ultrasound scan

If you can’t remember the first day of your last period or cannot pinpoint your date of ovulation, an ultrasound can help determine your due date. All women do not need an early ultrasound (before 13 weeks). Doctors recommend them only if your periods are irregular, you’re over 35 years or have a history of pregnancy complications or miscarriages.

An internal ultrasound or transvaginal ultrasound is performed between nine and 13 weeks of gestation. During this ultrasound, the technician measures the length of the fetus from crown to rump, which helps to estimate how old the fetus is and thus helps to calculate the due date.

The biparietal diameter scan is accurate in measuring gestational age from 14 to 20 weeks. During this scan, the technician measures the diameter of the baby’s skull. Ultrasound measurements for estimating the age of the fetus are more accurate during the early stages of the pregnancy. 

Why does the accuracy of the due date matter?

Only 5% of babies are born on their due date. Yet finding your due date is the first and the most important thing that your doctor does during your first prenatal visit. Having an accurate due date helps the doctors to monitor your pregnancy and your baby’s growth. Many of the tests done during pregnancy need to be done during a specific time frame. If the due date is not correct, the results of the tests may not be accurate.

The doctors can track your baby’s growth when they know your due date. For example, your doctor should be able to hear your baby’s heartbeat with a handheld Doppler by a specific week. The absence of a heartbeat can detect a problem.

Similarly, the doctors advise expecting mothers based on the progress of the pregnancy. For example, women are advised to take folate until the baby’s neural tube is developed, which is around 12 weeks. Knowing your due date helps you to understand how far your pregnancy has progressed and till when to take the supplements. 

Last weeks of pregnancy

The last few weeks of pregnancy are crucial for the growth of your baby. Their lungs and immune system are in the development stage until the very end of the pregnancy. Babies born before 39 weeks may have to be admitted to the NICU for the following complications –

  • They may need respiratory support, mechanical ventilation or intubation
  • They may need IV fluids or antibiotics
  • They may develop hypoglycemia (low blood sugar)

The risk of complications also starts increasing after more than 42 weeks of pregnancy.

  • The placenta might gradually stop being able to do its job correctly.
  • An infection might develop inside the womb.
  • Unexpected problems might arise during labor.
  • Childbirth can be difficult if the baby is too big.

Your doctor may recommend induction at this stage. Knowing your due date helps your doctor to decide when to slow down of halt pre-term labor and when to induce labor.

Pregnancy length range

Till fairly recently, doctor’s used to considered 37 weeks full term. However, research has shown that being born around 37 weeks can increase the risk of complications. Here’s how American College of Obstetricians and Gynaecologists (ACOG) now classify pregnancy lengths-

  • Early Term – Between 37 and 38 weeks
  • Full Term – Between 39 Weeks and 40 weeks, 6 days
  • Late-Term – Between 41 weeks and 41 weeks, 6 days
  • Post-term – 42 weeks and beyond

If you’re before the 39-week mark, inducing labor or scheduling C-section is not recommended unless you have preeclampsia or placenta previa.

Factors that affect the length of your pregnancy

The length of your pregnancy can be affected by many factors, including genetics, ethnicity, length of the menstrual cycle, and age of the mother. 

1. Genetic heritage

Recent research has shown that the genetic origin of both parents affects the length of the pregnancy. According to a study from Norway, the gestational age of the baby at birth increased, on average, by 0.58 days for each additional week in the father’s gestational age and 1.22 days for each extra week in the mother’s gestational age (1).

2. Ethnicity

The length of pregnancy also differs in people from different backgrounds. According to a 2014 study, women with black or Asian ethnic heritage have slightly shorter pregnancies (39 weeks) as compared to women with white heritage (2).

3.  Menstrual cycle

Not everyone’s menstrual cycle is the same length. If your menstrual cycle is longer than the average 28 days, you are likely to have longer than average pregnancies. The time it takes from fertilization until the embryo implants can also vary, and that also affects the length of the pregnancy (3).


4. Mother’s age

Older mothers tend to have longer pregnancies. Studies suggest that each year of the mother’s age adds on an average one day to the length of her pregnancy (4). The mother’s birth weight also makes a difference of one day for every 100 grams.

Final thoughts

Knowing your accurate due date is fundamental to good pregnancy care. Yet only 5% of births take place precisely on the estimated due date. A normal pregnancy lasts between 38 to 42 weeks. You can calculate your due date with the help of an online due date calculator by adding the first day of your last period or the date you conceived into the calculator.


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