How is a due date calculated?

Naegele Rule: add 280 days (40 weeks) to LMP. Ultrasound dating is more accurate for irregular cycles. Only 4% of babies are born on their exact due date. Full term is weeks 37-42.

What are the three trimesters of pregnancy?

First trimester: weeks 1-12 (all major organs form). Second: weeks 13-26 (baby grows rapidly, movements felt). Third: weeks 27-40 (rapid weight gain, birth preparation). Full term is weeks 37-42.

How often should I track my pregnancy dates?

Register with your midwife early. The dating scan at 11-14 weeks provides the most accurate due date. After that, regular antenatal appointments every 4 weeks until 28 weeks, then more frequently.

CConception date calculator | When Did I Conceive? Conception date calculator for couples

Choose Your Calculation Method

📅 Method 1 — Last Menstrual Period (LMP): Standard obstetric method. Conception typically occurs 14 days after your LMP (adjusted for your cycle length). Due date = 280 days from LMP.

First Day of Last Period

First day of bleeding, not spotting

Average Cycle Length

Adjusts estimated ovulation day

👶 Method 2 — From Due Date: Already know your due date? Work backwards. Conception is approximately 266 days (38 weeks) before your due date.

Your Due Date (EDD)

From your doctor, midwife, or scan

How was due date determined?

Helps assess accuracy

🔬 Method 3 — From Ultrasound: Most accurate method. Enter your scan date and the gestational age the sonographer recorded. Back-calculates your LMP and conception date.

Date of Ultrasound Scan

The date the scan was performed

Gestational Age at Scan

wks

days

Weeks + days shown on scan report

Estimated Conception Date

—

Pregnancy Progress

1st Trimester
Wks 1-12

2nd Trimester
Wks 13-26

3rd Trimester
Wks 27-40

📈 Your Pregnancy Timeline

✅ What to Do Now

Important steps for your pregnancy.

🔗 Related Pregnancy Tools

🥚

Ovulation Calculator

Fertile window tracker

📅

Period Calculator

Cycle tracking

⚖️

Pregnancy Weight Gain

Healthy gain tracker

👶

Baby Weight Percentile

After birth tracker

📏

Baby Height Predictor

Adult height estimate

🍼

Breast Milk Calculator

Storage guide

🦴

Calcium Calculator

Critical in pregnancy

💊

Vitamin B (Folate)

Neural tube protection

💧

Water Intake

Pregnancy hydration

✓

Content reviewed against ACOG pregnancy dating guidelines, NHS antenatal care standards, and WHO pregnancy recommendations. Last updated: July 2026.

How to Calculate Your Conception Date

Your conception date is the day the egg was fertilised — usually within 24 hours of ovulation. Three main methods estimate it: from your Last Menstrual Period (LMP) by adding 14 days (for a 28-day cycle), from your due date by subtracting 266 days (38 weeks), or from an ultrasound scan where the sonographer measures the baby and provides a gestational age. No method gives a single exact date — conception occurs within a window of several days around ovulation.

For most women, the conception date is not the same as the date of intercourse. Sperm can survive in the female reproductive tract for up to five days, meaning sex several days before ovulation can still result in pregnancy. The egg itself is only viable for 12–24 hours after ovulation. This is why conception is described as a window of several days rather than a single moment in time.

Method 1 — From Last Menstrual Period (LMP)

The LMP method is the standard approach used by obstetricians and midwives worldwide. The first day of your last menstrual period is taken as day 1 of pregnancy. Ovulation typically occurs around day 14 for a standard 28-day cycle. Adding 14 days to your LMP gives your estimated conception date; adding 280 days gives your due date.

The key limitation is the assumption of a 28-day cycle. In reality, cycles typically range from 21 to 35 days, and ovulation does not always fall at the midpoint. Our calculator adjusts for your actual cycle length. Use our Ovulation Calculator alongside this tool to better understand your fertile window.

Method 2 — From Your Due Date (Reverse Calculation)

If you already know your due date, work backwards to estimate conception. The calculation is: conception date = due date − 266 days (38 weeks). This works because the average pregnancy lasts 280 days from LMP, and conception occurs approximately 14 days after LMP, leaving 266 days from conception to delivery.

This is particularly useful for women who already know their due date from a doctor's appointment. The accuracy depends on how the due date was established — ultrasound-based due dates are more reliable than LMP-based ones for irregular cycles.

Method 3 — From Ultrasound Dating (Most Accurate)

First-trimester ultrasound (ideally 11–14 weeks) is the most accurate pregnancy dating method. The sonographer measures the crown-rump length (CRL) and determines gestational age within 5–7 days of accuracy. To back-calculate: subtract the gestational age in days from the scan date to get estimated LMP, then add 14 days to get the estimated conception date. The NHS and ACOG both recommend offering all pregnant women a dating scan in the first trimester.

Understanding the Fertile Window

The fertile window spans approximately six days: the five days before ovulation and ovulation day itself. For a standard 28-day cycle, this falls around days 10–15. For a 35-day cycle, around days 17–22. Sperm can survive up to five days in the fallopian tubes; the egg is viable for only 12–24 hours. Our Ovulation Calculator and Period Calculator can help you track this precisely.

Pregnancy Trimesters — What Happens When

First Trimester (Weeks 1–12)

All major organs form during this period. The heart begins beating around week 6. By week 10, the embryo becomes a fetus. This is also the period of highest miscarriage risk — approximately 80% occur in the first trimester, most due to chromosomal abnormalities.

Second Trimester (Weeks 13–26)

Morning sickness typically subsides and energy returns. Gender can often be identified on ultrasound around weeks 18–20. The baby begins to move (quickening) around weeks 16–20. Viability begins around week 24. Check your pregnancy weight with our Pregnancy Weight Gain Calculator.

Third Trimester (Weeks 27–40)

Rapid weight gain as the baby grows. Lungs mature in preparation for breathing. Birth is expected between weeks 37 and 42. Before 37 weeks is premature; after 42 weeks is post-term.

What to Do Once You Know Your Conception Date

Contact your GP or midwife to register your pregnancy and book your first antenatal appointment
Start folic acid immediately — 400 mcg daily. Critical for preventing neural tube defects. See our Vitamin B Complex Calculator for your folate needs
Book your dating scan — offered at 11–14 weeks for the most accurate due date
Stop alcohol and smoking completely — no safe level exists in pregnancy
Check your medications with your doctor — some are not safe in pregnancy
Optimise nutrition — increase calcium, iron, and folate. Use our Calcium Calculator and Water Intake Calculator

✅ Important: This calculator provides estimates based on standard obstetric calculations. For clinical pregnancy dating and care, always consult your GP, midwife, or obstetrician. First-trimester ultrasound is the most accurate dating method available.

Frequently Asked Questions

Three methods: (1) LMP method — add 14 days to your last period date; (2) From due date — subtract 266 days; (3) From ultrasound — subtract gestational age in days from scan date, then add 14 days. All three give estimates, not exact dates.

First-trimester ultrasound is most accurate (within 5-7 days). LMP-based calculation assumes a standard 28-day cycle with ovulation on day 14. Sperm survive up to 5 days, so conception occurs within a window rather than a single date.

Gestational age is counted from your LMP (standard medical measure). Conception age counts from fertilisation — approximately 2 weeks less. When a doctor says 10 weeks pregnant, the embryo is about 8 weeks old from conception.

Naegele's Rule: add 280 days (40 weeks) to your LMP. Only 4% of babies are born on their exact due date. Full term is weeks 37-42. Ultrasound dating is more accurate than LMP for irregular cycles.

Yes. Sperm survive up to 5 days. The fertile window is the 5 days before ovulation plus ovulation day — 6 days total. Stress or illness can shift ovulation timing, changing when the window occurs.

First trimester: weeks 1-12 (all major organs form). Second: weeks 13-26 (baby grows rapidly, movements felt, gender visible on scan). Third: weeks 27-40 (rapid weight gain, lungs mature, birth preparation). Full term is weeks 37-42.

Contact your GP or midwife. Start folic acid 400 mcg daily. Stop alcohol and smoking. Book your dating scan at 11-14 weeks. Check all medications with your doctor. Optimise calcium, iron, and folate intake.

Stress can delay or suppress ovulation, shifting the actual conception date later than standard calculations predict. If you experienced significant stress in the conception month, your ovulation may have occurred later than day 14.

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