Pregnancy Due Date Calculator

To use this pregnancy due date calculator, follow these steps:

1. Enter the year of your LMP. Use the year field to select the calendar year in which the first day of your last menstrual period fell. The calculator supports years from 2020 through 2030.
2. Select the month of your LMP. Choose the correct month from the dropdown menu. Accuracy here is essential because even a one-month error will shift your due date by approximately 30 days.
3. Enter the day of your LMP. Input the specific day of the month on which your last period began. If you are unsure of the exact date, use your best estimate and discuss with your healthcare provider for confirmation.
4. Adjust your cycle length if needed. The default cycle length is 28 days, which is the standard assumption behind Naegele's rule. If your cycles are consistently longer or shorter than 28 days, expand the advanced settings and enter your average cycle length. For example, if your cycles typically run 32 days, enter 32. This adjustment shifts your estimated ovulation date and consequently your due date. A longer cycle means ovulation occurred later, pushing the due date further out, while a shorter cycle means earlier ovulation and an earlier due date.
5. Review your results. The calculator instantly displays your estimated due date, the number of completed weeks and total days of pregnancy, your current trimester, and the number of days remaining until your due date.

Remember that this calculator provides an estimate. Your obstetrician or midwife may adjust the due date based on first-trimester ultrasound measurements, which can be more accurate than LMP-based calculations, particularly for women with irregular cycles.

A pregnancy due date calculator estimates when your baby is expected to be born based on the first day of your last menstrual period (LMP). The calculation relies on Naegele's rule, a method developed by German obstetrician Franz Karl Naegele in the early 19th century and still used as the standard approach by healthcare providers worldwide to determine an estimated due date (EDD). Naegele's rule works by adding 280 days, or 40 weeks, to the first day of your LMP, which assumes that ovulation and conception occurred approximately 14 days into a standard 28-day menstrual cycle.

It is important to understand the distinction between gestational age and fetal age. Gestational age is counted from the first day of the LMP and is the standard measure used in clinical practice. Fetal age, also called embryonic or conceptional age, begins at the estimated date of conception and is roughly two weeks shorter than gestational age. When a doctor says you are 12 weeks pregnant, that refers to gestational age, meaning the baby has actually been developing for about 10 weeks.

Despite the precision of these calculations, only about 5 percent of babies are born on their exact due date. Most deliveries occur within a two-week window on either side of the EDD, between 38 and 42 weeks of gestation. This variability is perfectly normal and depends on factors such as genetics, the mother's health, and whether this is a first pregnancy.

Pregnancy is divided into three trimesters, each with distinct developmental milestones. The first trimester spans weeks 1 through 12 and is when the baby's major organs begin to form. The second trimester covers weeks 13 through 26 and is often considered the most comfortable period, as nausea typically subsides and the baby's movements become noticeable. The third trimester runs from week 27 through week 40, during which the baby gains most of its weight and the body prepares for labor and delivery.

Early prenatal care is critical for a healthy pregnancy. Scheduling your first prenatal visit as soon as you suspect you are pregnant allows your healthcare provider to confirm the pregnancy, estimate the due date with an early ultrasound, screen for potential complications, and begin recommended supplements such as folic acid. Early ultrasound dating in the first trimester is generally more accurate than LMP-based calculations because it measures the embryo directly rather than relying on assumptions about cycle regularity.

The due date is calculated using Naegele's rule with an optional cycle length adjustment. The core formulas are as follows:

• Estimated Due Date (EDD) = LMP Date + 280 days + (Cycle Length − 28) days
• Days Pregnant = Today's Date − LMP Date (measured in calendar days)
• Weeks Pregnant = Days Pregnant ÷ 7, rounded down to the nearest whole number
• Days Remaining = Estimated Due Date − Today's Date

The trimester boundaries are determined by the number of completed weeks:

• First Trimester: Week 1 through Week 12 (days 1 through 84)
• Second Trimester: Week 13 through Week 26 (days 85 through 182)
• Third Trimester: Week 27 through Week 40 (days 183 through 280)

The cycle length adjustment accounts for the fact that ovulation does not always occur on day 14. In a standard 28-day cycle, ovulation is assumed to happen on day 14. If your cycle is 32 days, ovulation likely occurs around day 18, which is 4 days later. The formula adds this difference to the standard 280 days, resulting in a total gestation estimate of 284 days. Conversely, a 25-day cycle subtracts 3 days, yielding 277 days.

Variable definitions:

Example 1: Standard 28-day cycle. A woman's last menstrual period began on June 1, 2025, and she has a regular 28-day cycle. Using Naegele's rule: EDD = June 1, 2025 + 280 days + (28 − 28) days = June 1, 2025 + 280 days = March 8, 2026. On September 15, 2025, she would be 106 days pregnant, which equals 15 weeks and 1 day. She would be in the second trimester with approximately 174 days remaining until her due date. Her baby's organs are now fully formed and the baby is beginning to move.

Example 2: Longer 32-day cycle. A woman's LMP was May 15, 2025, and her average cycle length is 32 days. The cycle adjustment adds 4 extra days because ovulation likely occurred on day 18 rather than day 14. EDD = May 15, 2025 + 280 days + 4 days = May 15, 2025 + 284 days = February 23, 2026. By October 1, 2025, she would be 139 days pregnant, or 19 weeks and 6 days, placing her in the second trimester with 145 days remaining. The longer cycle means conception happened later, so the pregnancy is slightly less advanced than it would be with a 28-day cycle.

Example 3: Shorter 25-day cycle. A woman's LMP was July 10, 2025, and her average cycle is 25 days. The adjustment subtracts 3 days because ovulation likely occurred on day 11 instead of day 14. EDD = July 10, 2025 + 280 days − 3 days = July 10, 2025 + 277 days = April 13, 2026. On December 1, 2025, she would be 144 days pregnant, which equals 20 weeks and 4 days. She is in the second trimester with approximately 133 days left. The shorter cycle means the baby was conceived earlier, so development is slightly further along compared to a 28-day cycle assumption.