Conception Calculator | OmniCalculator

  • Adam
  • November 1, 2025

Free conception calculator to estimate due date, conception date, and pregnancy timeline. Calculate based on LMP, ovulation, or conception date using Naegele’s rule.

👶 💕 👶

Conception Calculator

Calculate your conception date, estimate your due date, and track your pregnancy timeline based on your menstrual cycle.

📋 Quick Navigation

👶 Calculate your conception date and due date! 💕
First day of your last menstruation
21-35 days (average 28)
typically day 12-16
Date of conception (ovulation or intercourse)
Date egg was released
21-35 days
typically 12-16
Estimated Due Date
TBD
approximately 40 weeks from LMP

Conception Date

—
likely date

Due Date

—
40 weeks

Fertile Window

6
days

Pregnancy Length

280
days

📊 Detailed Timeline

Event Estimated Date Weeks Notes
LMP — Week 0 First day of last period
Ovulation — Week 2 Egg released
Conception — Week 2 Fertilization likely
Implantation — Week 3-4 Embryo implants
Positive Test — Week 4 hCG detectable
Due Date — Week 40 Estimated delivery

🤰 Pregnancy Timeline

Trimester 1 (Weeks 1-12)

Week 2-4: Ovulation, conception, and implantation occur. Embryonic development begins. Week 5-8: Heart begins beating, neural tube forms, limbs develop. Week 9-12: All major organs forming, fetal movement begins, risk of miscarriage decreases significantly after week 12.

Trimester 2 (Weeks 13-26)

Week 13-16: Baby can hear, genetic testing available, fetal movements become noticeable. Week 17-20: Detailed ultrasound performed, sex often determined. Week 21-26: Baby gains weight rapidly, eyes open, lungs develop, baby may survive if born now with medical help (viability point).

Trimester 3 (Weeks 27-40)

Week 27-30: Baby gains more weight, brain development continues. Week 31-34: Baby moves into head-down position, birth practitioner checks fetal position. Week 35-40: Baby drops into pelvis, final weight gain, labor can begin anytime after week 37 (term).

💡 Note: Only about 5% of babies arrive on their exact due date. Most arrive within 2 weeks before or after. Healthcare providers typically induce labor at 42 weeks if baby hasn't arrived.

📐 Calculation Methods

Last Menstrual Period (LMP) Method

The most common and straightforward method. Pregnancy is calculated as 280 days (40 weeks) from the first day of your last menstrual period, even though conception typically occurs 2 weeks later. This method assumes a regular 28-day cycle with ovulation on day 14.

Conception Date Method

If you know your exact conception date (from fertility tracking or assisted reproduction), add 266 days (38 weeks) to get your due date. This is more accurate than LMP for irregular cycles or those who conceived through assisted reproduction.

Ovulation Date Method

If you tracked ovulation using temperature, cervical mucus, or ovulation predictor kits, add 266 days to your ovulation date. Most accurate for those who precisely track ovulation.

Naegele's Rule

A simple calculation method: Add 7 days to LMP, subtract 3 months, then add 1 year. Works well for 28-day cycles but less accurate for those with longer or shorter cycles.

📌 Accuracy: All methods assume a typical 28-day cycle. If your cycle is longer or shorter, your due date may shift by several days. Ultrasound dating, especially in the first trimester, is the most accurate (±3-5 days error).

🧮 Calculation Formulas

Due Date from LMP (Naegele's Rule)

Due Date = LMP + 280 days
Or: Due Date = LMP + 40 weeks
Or: Due Date = LMP + 9 months and 7 days

Alternative (Naegele's Rule):
1. Add 7 days to LMP
2. Subtract 3 months
3. Add 1 year (if needed)

Example: LMP = Nov 2, 2025
Due Date = Nov 2 + 280 days = Aug 9, 2026

Conception Date Calculation

Conception Date = LMP + Ovulation Day

For 28-day cycle (ovulation day 14):
Conception ≈ LMP + 14 days

Example: LMP = Nov 2
Conception ≈ Nov 2 + 14 = Nov 16

Due Date from Conception Date

Due Date = Conception Date + 266 days
Or: Due Date = Conception Date + 38 weeks

Example: Conception = Nov 16, 2025
Due Date = Nov 16 + 266 = Aug 9, 2026

Fertile Window Calculation

Fertile Window Start = Ovulation Day − 5 days
Fertile Window End = Ovulation Day

For 28-day cycle:
Fertile Window = Days 9-14 from LMP (6 days total)

For 35-day cycle (ovulation day 21):
Fertile Window = Days 16-21 from LMP (6 days total)

Weeks of Pregnancy at Any Date

Weeks Pregnant = (Current Date − LMP) / 7 days

Example: Current date Aug 9, 2026; LMP Nov 2, 2025
Days elapsed = 280 days
Weeks = 280 / 7 = 40 weeks (at due date)

❓ Frequently Asked Questions

Can my due date change during pregnancy? +
Yes, absolutely. Due dates are typically adjusted based on ultrasound dating, especially during first and second trimester scans. Early ultrasounds (before 12 weeks) can narrow the due date to within 3-5 days. Later scans may shift the date by days or weeks based on fetal measurements. Your healthcare provider will give the most accurate estimate.
What if I don't know my exact LMP? +
If you don't know your exact LMP but know your conception/ovulation date, use the conception method (add 266 days). If unsure of both, an early ultrasound (before 12 weeks) is your best option for accurate dating. Your doctor can estimate LMP backwards from ultrasound measurements.
How accurate is the due date? +
Accuracy depends on the method: LMP with regular cycles ±2 weeks, first trimester ultrasound ±3-5 days, second trimester ultrasound ±1-2 weeks. Only 5% of pregnancies result in delivery on the exact due date. 90% of babies arrive within 2 weeks of the due date.
When can I take a pregnancy test? +
Earliest reliable pregnancy tests: 10-14 days after conception (or about 24-28 days after LMP). Tests detect hCG hormone that appears after implantation (6-12 days post-conception). Home tests are most accurate after missed period. For earliest detection, use sensitive tests and test with first morning urine.
What's the difference between gestational age and fetal age? +
Gestational age is measured from LMP (even though baby is conceived 2 weeks later), so at "week 4" gestational age, baby is actually only 2 weeks old. Fetal age (developmental age) is 2 weeks behind gestational age. Doctors use gestational age for medical purposes and development tracking.
Can I get pregnant before my fertile window? +
Very unlikely, but technically possible. Sperm can live up to 5 days, so intercourse up to 5 days before ovulation can result in pregnancy. However, the closer to ovulation, the higher the conception rate. The fertile window represents when pregnancy is most likely, not when it's impossible outside.
What if I have irregular periods? With irregular periods, LMP calculations are less reliable. Track your cycle for several months to find your average length. If ovulation varies significantly, use ovulation tracking methods or get early ultrasound dating. Your healthcare provider can help establish a more accurate due date.

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