How accurate is the due date calculation?

Naegele's Rule provides an estimate — only about 5% of babies are born exactly on their due date. Most births occur within two weeks before or after the estimated date. An ultrasound in the first trimester provides the most accurate dating.

What is Naegele's Rule?

Naegele's Rule estimates the due date by adding 280 days (40 weeks) to the first day of the last menstrual period. This assumes a standard 28-day cycle. Our calculator adjusts the date by the difference between your actual cycle length and 28 days.

What if my cycle is not 28 days?

The calculator adjusts the due date for non-standard cycle lengths. For each day your cycle is longer than 28 days, the due date is pushed forward one day, and vice versa. For example, a 32-day cycle shifts the due date 4 days later.

Should I rely solely on this calculator for medical decisions?

No. This tool is for informational purposes only. Always consult your healthcare provider for accurate pregnancy dating, especially using first-trimester ultrasound which remains the gold standard for confirming gestational age.

Pregnancy Due Date Calculator

What is a Pregnancy Due Date Calculator?

A pregnancy due date calculator estimates the date your baby is expected to be born, based on the first day of your last menstrual period (LMP) and your average menstrual cycle length. The most widely used method is Naegele’s Rule, developed by the German obstetrician Franz Karl Naegele in the 19th century.

Naegele’s Rule works by adding 280 days (40 weeks) to the first day of your last period. This assumes a standard 28-day menstrual cycle. Because not all cycles are exactly 28 days, our calculator adjusts the due date by adding or subtracting the difference between your actual cycle length and 28 days.

Formula:

Adjusted due date = LMP + 280 days + (cycle length − 28) days

For example, if your cycle is 32 days, the calculator adds 4 extra days to the standard 280-day estimate, resulting in a due date 284 days after your LMP.

Gestational age — how far along you are in pregnancy — is measured from the first day of your LMP, not from conception. This is because ovulation and conception dates are difficult to pinpoint precisely, whereas the start of the last period is usually known.

How to Use the Pregnancy Due Date Calculator

Enter your LMP date: Input the first day of your last menstrual period in the format YYYY-MM-DD.
Enter your cycle length: Provide the average number of days in your menstrual cycle (21–35 days). If unsure, use 28 as the default.
Enter today’s date: This is used to compute your current gestational age and the number of days until your due date.
Read your results: The calculator shows your estimated due date, current gestational age in weeks and days, which trimester you are in, and days remaining until your due date.

Keep in mind that this is an estimate. Your healthcare provider may use ultrasound measurements to refine the due date, particularly in the first trimester.

Examples

Example 1: Standard 28-Day Cycle

LMP: January 1, 2024
Cycle length: 28 days
Due date calculation: January 1, 2024 + 280 days = October 7, 2024
At 10 weeks (70 days after LMP): gestational age = 10 weeks 0 days, in first trimester, 210 days remaining

Example 2: Longer 32-Day Cycle

LMP: January 1, 2024
Cycle length: 32 days (4 days longer than standard)
Adjustment: +4 days
Due date = January 1, 2024 + 284 days = October 11, 2024
This reflects that ovulation likely occurred around day 18 of the cycle rather than day 14

Example 3: Shorter 24-Day Cycle

LMP: March 15, 2024
Cycle length: 24 days (4 days shorter than standard)
Adjustment: −4 days
Due date = March 15, 2024 + 276 days = December 16, 2024

FAQ

How accurate is the estimated due date? Naegele’s Rule is only an estimate. Research shows that only about 4–5% of babies are born on their predicted due date. Most births occur within two weeks before or two weeks after the estimated date. First-trimester ultrasound measurements of crown-rump length provide the most accurate gestational age assessment and are used to confirm or adjust the Naegele estimate.

What are the trimester boundaries? Pregnancy is divided into three trimesters based on gestational age (counted from the LMP):

First trimester: Weeks 1–13 (days 1–97) — early development, organ formation
Second trimester: Weeks 14–27 (days 98–195) — fetal growth, movement begins
Third trimester: Weeks 28 to birth (days 196+) — rapid weight gain, preparation for birth

What if I don’t know my LMP date? If you don’t know your last period date, your doctor can estimate gestational age using first-trimester ultrasound, which measures the crown-rump length of the fetus. This method is considered the gold standard for dating pregnancies when the LMP is uncertain.

Does cycle length affect when I ovulate? Yes. In a 28-day cycle, ovulation typically occurs around day 14. In a 32-day cycle, ovulation usually occurs around day 18. The baby’s due date depends on when conception occurred, which is related to ovulation timing. Adjusting for cycle length accounts for this variation in ovulation timing.

Should I rely solely on this calculator? No. This tool provides an educational estimate. Always consult your obstetrician or midwife for accurate pregnancy dating, prenatal care planning, and monitoring. Ultrasound remains the most reliable method for confirming gestational age.