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Naegele’s Rule to Calculate Delivery Date

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Naegele's rule calculates the estimated due date for delivery based on the last menstrual period date. Please read the article to know how it is done.

Medically reviewed by

Dr. Arjun Chaudhari

Published At March 9, 2023
Reviewed AtMarch 22, 2024

What Is Naegele's Rule?

Naegele's rule is attributed to the German obstetrician Franz Naegele (1778-1851). Pregnancy usually lasts about 280 days, which is 40 weeks, calculated from the first day of the last menstrual period (LMP). Therefore, his rule adds nine months and seven days to the first day of the last menstrual period.

What Is an Estimated Date of Delivery (EDD) in Pregnancy?

The estimated delivery date is the anticipated date on which the mother would deliver the baby. It is also known as the due date. The estimated date of delivery is calculated at the first prenatal examination.

The benefits of calculating the due date are-

  • It helps the healthcare provider to monitor the baby's growth more accurately.

  • It provides a timeline for necessary tests and diagnostics throughout the pregnancy.

  • Mentally prepares pregnant ladies for upcoming changes in their bodies and baby birth.

Can a Due Date Change Over Time?

The due date is one of the first things calculated during pregnancy. The due date guides the doctor in providing proper prenatal care, testing, and the baby's birth. That is why the due date must be as accurate as possible. The calculated due date is almost always correct and stays the same. However, in the following cases, the doctor might alter the date-

  • Abnormal fetal heart tones.

  • Uterine abnormalities.

  • Twin pregnancy.

  • Incorrect calculation of the fundus height (top of the uterus).

  • Excessive maternal weight.

  • Length of the menstrual cycle.

  • Abdominal circumference.

How to Calculate Due Date Using Naegele’s Rule?

Naegele’s Rule Formula-

Due date of delivery = First day of last menstrual cycle + 9 months + 7 days.

  • The Naegele's rule calculator calculates the women's due date of pregnancy based on the first day of the last menstrual cycle.

  • Based on the first day of the last menstrual cycle, the due date for the pregnancy is estimated by adding nine months to the date plus seven days.

  • An alternate way of calculating is to add a year to the date of the first day of the last menstrual cycle, subtract three months, and then add seven days to it.

  • Naegele's rule calculator also calculates leap years accurately.

Example of calculating the estimated due date of pregnancy using Naegele's rule-

  • If a woman had the first day of her last menstrual period (LMP) to be 4 April 2016-

    • + 1 year = 5 April 2017.

    • -3 months = 5 January 2017.

    • +7 days = 11 January 2017.

    • The expected date of delivery will be 11 January 2017.

280 days after the start of the last menstrual period is calculated by checking the day of the week of the last menstrual period and adjusting the calculated date to land on the same day of the week. However, the calculation method only sometimes results in 280 days because all calendar months are different lengths.

How Accurate Are the Due Date Calculating Formulas?

No estimated due date calculating formulas nor ultrasound scans are 100 percent accurate. All complications can occur during pregnancy, making it impossible to calculate the precise estimated delivery date. Naegele's rule assumes a woman's menstrual period is 28 days, which is the average cycle length of the menstrual period. But this is only sometimes correct; the menstrual cycle can be longer or shorter. Thus, Naegele's rule is an approximator that gives a due date near the target.

What Happens Past Due Date?

The due date is just an estimate; very few women give birth that day. If the due date is passed, there is nothing to worry about. A term pregnancy can be anywhere from 37 weeks to 42 weeks of gestation.

  • It is called late-term pregnancy if it lasts from 41 weeks to 42 weeks.

  • If the pregnancy lasts 42 weeks or longer, it is considered post-term or past due.

If a woman is overdue and still pregnant, she might feel anxious, uncomfortable, and tired of being pregnant. They must be ready and alert for the signs of labor. The doctor might recommend inducing labor if the pregnancy goes past 41 weeks. The doctor will make sure that the baby is thriving properly and might recommend tests like-

  • A nonstress test (NST) monitors the baby's heart rate.

  • A biophysical profile (ultrasound) to monitor amniotic fluid, heart rate, breathing, movement, and muscle tone.

What Are the Risks Associated with Overdue Pregnancy?

  • Most overdue pregnancies cause no complications for the mother and the baby. However, past-due babies might have long, thin limbs, dry, peeling skin, and longer hair and nails. They are often very alert at birth.

  • Some overdue pregnancies might have the following risks-

  • Stillbirth.

  • Macrosomia (baby size larger than average).

  • Postmaturity syndrome, or dysmaturity (neurodevelopmental complications).

  • Meconium (baby's first bowel movement) in the baby's lungs. It can cause breathing problems after birth.

  • Oligohydramnios (low amniotic fluid).

When Naegele's Rule Can Not Be Used for Calculating Due Date?

Naegele's rule cannot be used if the pregnant woman cannot recall the first day of her last menstrual period or have irregular menstrual cycles. In such cases, the due date is calculated based on ultrasounds.

What Are Some Other Ways to Calculate the Due Date?

  • Pregnancy wheel - Doctors can use a pregnancy wheel to calculate the estimated due date. The LMP (last menstrual period) is located on the wheel and lined up with the indicator to get the due date.

  • If the last menstrual period date is unknown - If a woman does not remember her last date, the doctor will provide an estimated due date based on the ultrasound impression.

  • Irregular periods - If a woman has irregular periods, the pregnancy wheel can still be used but with some adjustments according to the length of the menstrual cycle.

  • Ultrasound - The first date is always calculated using the date of the last menstrual period, and the second date is calculated based on the ultrasound measurements. These dates can be different from each other. Due date predictions are more accurate earlier in pregnancy. Later in pregnancy, the ultrasounds are more helpful in determining fetal growth, not the age of the fetus.


Naegele's rule is an excellent and effective way to calculate the due date in pregnancy, but it should be considered only a guideline and not a definite date.

Frequently Asked Questions


How Does Naegele's Rule Help in the Calculation of EDD (Expected Date of Delivery)?

Naegele's rule entails a simple calculation -
 - The beginning day of the last menstrual period (LMP) is added by seven; then, three months are subtracted. The year is changed accordingly (if necessary).
 - For example- If a person's LMP was April 1, 2010, add seven days (April 8, 2010). Take three months out (January 8, 2010). Change the year. Thus, here EDD will be January 8, 2011.


How Reliable Is the Naegele's Rule?

No predicted due date calculating formula is 100 % accurate, and even ultrasound tests are not entirely correct. Pregnancy difficulties of all kinds can arise, making it impossible to determine the exact projected delivery date. Naegele's rule presupposes that a woman's menstrual period would last for 28 days, which is the average menstrual cycle length. However, the menstrual cycle might be longer or shorter. Thus this is not always accurate. Therefore, Naegele's rule only provides an approximate due date close to the target.


Why Did Naegele's Rule Get an Extra Week by Adding Seven Days?

To determine the anticipated due date, Naegele's rule, which was developed by a German obstetrician, subtracts three months and adds seven days. The obstetrician should obtain a thorough menstrual history, including the frequency, flow, length, and use of hormonal contraceptives in the past. Then, seven days are added to track the ovulation of the female. It is based on an average ovulation cycle of 28 days.


Why Is the Naegele's Rule Applied?

The Naegele's Rule Calculator determines a woman's due date for pregnancy based on the first day of her most recent menstrual cycle. The estimated due date for the pregnancy is calculated by adding nine months and seven days to the first day of the most recent menstrual cycle a woman experiences.


How Is Naegele's Rule Affected?

The rule is based on a cycle duration of 28 days, with ovulation and fertilization occurring on day 14 of the cycle. Therefore, it is less reliable for women with irregular menstrual cycles and when ovulation occurs earlier or later in the cycle.


How To Modify Naegele's Rule?

The suggested modified Naegele's rule should be as follows- 
The first day of LMP is added with seven days, nine months, and correction value, adding or deducting the number of days that are greater than or smaller than the conventional 28-day cycle.


When Should the Naegele's Rule Not Be Applied?

Naegele's formula can not be used if the patient can not remember the first day of her last menstrual period or if she has irregular cycles.
Source Article IclonSourcesSource Article Arrow
Dr. Arjun Chaudhari
Dr. Arjun Chaudhari

Obstetrics and Gynecology


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