Due Date Calculator

Introduction

Maria, a 29-year-old accountant in Chicago, sees two pink lines on a pregnancy test on July 10, 2026. Her mind immediately jumps to one question: when is the baby due? She remembers her last period started on June 28. She pulls up this calculator, enters that date, and learns her estimated due date is April 4, 2027. She calls her doctor to schedule her first prenatal appointment.

Only about 5% of babies arrive on their exact due date. Most arrive within two weeks before or after. The due date is still valuable. It anchors your entire prenatal care timeline, from when to schedule your glucose tolerance test (24 to 28 weeks) to when your doctor will start monitoring for Group B Streptococcus (35 to 37 weeks). The American College of Obstetricians and Gynecologists (ACOG) reaffirmed its due date estimation guidelines in 2025, confirming that first-trimester ultrasound is the most accurate method for establishing gestational age.

Inputs Required

• Last Menstrual Period (LMP): The first day of your last period
• Cycle Length: Your average menstrual cycle length in days (default is 28)

Outputs Provided

• Estimated Due Date: Your baby's expected arrival date
• Estimated Conception Date: When your baby was likely conceived
• Delivery Window: The range of dates when delivery is most likely
• Days Remaining: How many days until your due date

How the Calculation Works

The due date calculator uses Naegele's rule, which adds 280 days (40 weeks) to the first day of your last menstrual period. This method is based on the average human pregnancy lasting 280 days from the LMP.

The calculator also estimates your conception date by adding your cycle length minus 14 days to your LMP. This accounts for the fact that ovulation typically occurs about 14 days before your next period. The delivery window extends two weeks before and after your due date, as this is when most babies arrive.

ACOG recommends that if your LMP-based due date differs from a first-trimester ultrasound by more than 7 days, the ultrasound date should be used. Before 9 weeks, a discrepancy of more than 5 days triggers a revision. According to StatPearls pregnancy dating reference, first-trimester ultrasound has an accuracy of plus or minus 5 to 7 days, compared to plus or minus 7 to 10 days in the early second trimester and 21 to 30 days in the third trimester.

How to Use the Calculator

1. Enter the first day of your last menstrual period
2. Adjust your cycle length if it differs from the standard 28 days
3. View your estimated due date and conception date
4. Note the delivery window when your baby is most likely to arrive
5. Check how many days remain until your due date

To track your pregnancy week by week, try our Pregnancy Calculator. If you are trying to conceive, our Ovulation Calculator can help you find your fertile window.

Example Calculations

Example 1: Standard 28-Day Cycle

If your last menstrual period was June 28, 2026, with a 28-day cycle:

• Due date: April 4, 2027
• Conception date: July 12, 2026
• Delivery window: March 21 to April 18, 2027
• Pregnancy duration: 40 weeks

Example 2: Longer 35-Day Cycle

If your last menstrual period was June 20, 2026, with a 35-day cycle:

• Due date: April 10, 2027 (adjusted for later ovulation)
• Conception date: July 11, 2026 (cycle length minus 14 = day 21)
• Delivery window: March 27 to April 24, 2027
• Note: Longer cycles push ovulation later, which shifts both conception and due date

Real-World Scenarios

Planning Maternity Leave

Aisha, a 33-year-old project manager in Seattle, calculates her due date as April 4, 2027. She discusses maternity leave timing with her HR department in January. Under the Family and Medical Leave Act, she is entitled to 12 weeks of unpaid leave, but her company offers 16 weeks of paid leave. She plans to stop working on March 20, two weeks before her due date, to prepare. She arranges for her colleague to take over her client accounts starting March 15.

Irregular Cycles and Ultrasound Adjustment

Rachel, a 26-year-old nurse in Denver, has irregular cycles ranging from 30 to 45 days. She enters her LMP of June 15 with a 35-day cycle length and gets a due date of April 5, 2027. At her 8-week ultrasound on August 10, the crown-rump length measurement gives a due date of April 12. Because the discrepancy is more than 5 days at less than 9 weeks, her doctor revises the due date to April 12 per ACOG guidelines. This is a common scenario for women with irregular cycles.

IVF and Known Conception Date

Priya, a 38-year-old architect in Boston, conceived through in vitro fertilization. She knows the exact embryo transfer date: July 1, 2026. For IVF patients, the due date is calculated from the transfer date plus 266 days (for a day-5 embryo transfer), giving her a due date of March 24, 2027. The LMP-based calculator gives a close approximation, but IVF dating is more precise because the conception date is known exactly. Her doctor uses the IVF-derived date as the primary reference.

Why This Calculation Matters

Your due date is essential for coordinating prenatal care. Healthcare providers use it to schedule important screening tests like the glucose tolerance test (typically around 24 to 28 weeks) and Group B Streptococcus testing (around 35 to 37 weeks). Your due date also helps your doctor monitor whether your baby is growing appropriately.

If labor does not begin by 42 weeks, your healthcare provider may recommend induction to reduce health risks. ACOG considers a pregnancy without an ultrasound confirming the due date before 22 weeks to be suboptimally dated, which affects how the doctor manages late-term delivery decisions. For tracking recommended weight gain during your pregnancy, use our Pregnancy Weight Gain Calculator.

Common Mistakes to Avoid

• Using the wrong date: Always use the first day of your last period, not the last day or any other date. The first day of bleeding is day 1 of your cycle
• Forgetting to adjust for cycle length: If your cycle is longer or shorter than 28 days, adjust the calculator. A 35-day cycle shifts ovulation and the due date by about a week compared to a 28-day cycle
• Treating the due date as exact: Only about 5% of babies arrive on their exact due date. Plan for a range of dates within the delivery window
• Not confirming with your doctor: Always verify your due date with your healthcare provider, especially if you are unsure of your LMP. First-trimester ultrasound is more accurate than LMP-based calculation
• Ignoring ultrasound dating: If your ultrasound shows a due date more than 7 days different from your LMP calculation in the first trimester, ACOG guidelines say the ultrasound date should be used

Limitations of This Calculator

This calculator uses Naegele's rule, which assumes a regular 28-day cycle with ovulation on day 14. Women with irregular cycles, those who were taking hormonal birth control, or those who conceived through assisted reproductive technology will get less accurate results. The calculator does not account for first-trimester ultrasound measurements, which ACOG considers the gold standard for pregnancy dating. It does not replace professional medical advice. Always confirm your due date with your obstetrician or midwife at your first prenatal visit.

Authoritative Research and Resources

• ACOG Committee Opinion No. 700: Methods for Estimating the Due Date - The official guideline from the American College of Obstetricians and Gynecologists, reaffirmed in 2025. Establishes that first-trimester ultrasound (up to 13 6/7 weeks) is the most accurate method for gestational age dating, with an accuracy of plus or minus 5 to 7 days.
• Pregnancy Dating - StatPearls (NIH) - A peer-reviewed medical reference from the National Center for Biotechnology Information. Details Naegele's rule, ultrasound dating accuracy by trimester, and ACOG discrepancy thresholds for revising the estimated due date.
• Sonography Assessment of Gestational Age - StatPearls (NIH) - Covers crown-rump length measurement methodology, accuracy by gestational age, and the 22-week cutoff for suboptimally dated pregnancies per ACOG guidelines.