What are the three trimesters and what gestational weeks do they cover?

The first trimester covers weeks 1-12 and includes fertilization, implantation, embryo formation, and all major organ development. The second trimester covers weeks 13-26 and is typically the most comfortable period, with fetal movements beginning around week 20. The third trimester covers weeks 27-40 and focuses on weight gain, lung maturation, and preparation for birth.

What does it mean if the calculator shows a past due date?

If your EDD is in the past, the results section shows how many days ago the EDD was. This can happen if you entered a completed pregnancy's due date to look up the historical conception date. The gestational age result will show post-term for dates beyond 42 weeks.

Conception Calculator

Q: What if my due date from this calculator differs from my doctor's date?

Small differences (1-2 weeks) are normal and usually reflect cycle length variations from the assumed 28 days, or uncertainty about the exact ovulation date. Your doctor's due date, especially if set by a first-trimester ultrasound, is more reliable than LMP-based calculations and should be used for all medical planning.

Find your estimated conception date from a due date or last menstrual period, with gestational age, trimester, and fertile window.

🌱 What is a Conception Calculator?

A conception calculator estimates the date when fertilization most likely occurred, based on either your estimated due date (EDD) or your last menstrual period (LMP) and cycle length. Conception happens when a sperm successfully fertilizes an egg during ovulation, typically in the middle of the menstrual cycle. Because the exact moment of fertilization is rarely observed directly, conception dates are always estimates derived from surrounding dates that are easier to know: the LMP or the due date.

Medical professionals use two main anchor points to estimate conception. The first is the LMP method: since ovulation reliably occurs approximately 14 days before the next period, the conception date can be approximated as LMP plus (cycle length minus 14). The second is the EDD method: working backwards from a known or estimated due date, subtracting 266 days (the average length of fetal development from conception to birth) gives the approximate conception date.

Knowing your conception date is useful in several situations. Parents curious about the specific timing of their pregnancy, couples using timed intercourse for conception, and anyone trying to understand their fertile window all benefit from this calculation. It is also helpful for retrospectively understanding whether a positive test result aligns with expected timing, or for reviewing past pregnancies. In legal and administrative contexts, conception date estimates occasionally support paternity timelines, though such determinations require medical confirmation.

This calculator operates in two modes. From Due Date mode takes a known EDD and returns the estimated conception date, estimated LMP, current gestational age, and trimester. From LMP mode takes the first day of the last period and cycle length, then returns the ovulation (conception) date, the 6-day fertile window, the estimated due date, and current gestational age. Both modes provide accurate estimates for regular cycles; for irregular cycles, an early ultrasound gives more reliable results.

Medical disclaimer: This calculator is for informational and educational purposes only. Always consult your obstetrician, midwife, or healthcare provider for medical dating, pregnancy confirmation, and clinical advice.

📐 Formula

From Due Date:

Conception Date = EDD − 266 days

EDD = Estimated Due Date (40 weeks from LMP, or 38 weeks from conception)

266 days = 38 weeks, the standard fetal development period from conception to birth

Est. LMP = EDD − 280 days (40 weeks before the due date)

From LMP:

Conception Date = LMP + (Cycle Length − 14)

LMP = First day of the Last Menstrual Period

Cycle Length − 14 = Days from LMP to ovulation (luteal phase is fixed at 14 days)

Fertile Window = Conception Date − 5 days → Conception Date

EDD = LMP + 280 + (Cycle Length − 28)

280 days = 40 weeks (standard Naegele's rule for a 28-day cycle)

The cycle length adjustment shifts the EDD later for longer cycles and earlier for shorter cycles

Example: LMP = April 1, Cycle = 35 days. Ovulation = April 1 + 21 = April 22. EDD = April 1 + 287 = December 13.

📖 How to Use This Calculator

Steps

Choose your mode - click "From Due Date" if you already know your EDD. Click "From LMP" if you know when your last period started.

Enter your date - for From Due Date, type or pick your EDD. For From LMP, enter the first day of your last period and set your average cycle length using the slider (default is 28 days).

Click Calculate - the results instantly show your estimated conception date, due date, current gestational age, trimester, and (in LMP mode) the 6-day fertile window.

Share or save - use the Copy link button to bookmark the exact calculation, or WhatsApp to share with your partner or family.

💡 Example Calculations

Example 1 - Christmas Due Date (From Due Date Mode)

A mother has an EDD of December 25, 2026. She wants to find when conception most likely occurred.

EDD: December 25, 2026

Conception Date = EDD − 266 days = December 25, 2026 − 266 = April 3, 2026

Est. LMP Date = EDD − 280 days = December 25 − 280 = March 20, 2026

On May 17, 2026: Days from est. LMP = 58 days → Week 8 + 4 days (First Trimester)

Days to EDD = December 25 − May 17 = 222 days remaining

Conception Date: April 3, 2026 | Est. LMP: March 20, 2026 | Week 8+4 | First Trimester

Try this example →

Example 2 - Standard 28-Day Cycle (From LMP Mode)

A couple trying to conceive wants to know their fertile window and due date.

LMP: April 1, 2026 | Cycle: 28 days

Ovulation (Conception) = April 1 + (28 − 14) = April 1 + 14 = April 15, 2026

Fertile Window = April 10 to April 15, 2026 (6 days: 5 before + ovulation day)

EDD = April 1 + 280 = January 6, 2027

On May 17, 2026: 46 days from LMP → Week 6 + 4 days (First Trimester)

Conception: April 15, 2026 | Fertile Window: April 10-15 | EDD: January 6, 2027

Try this example →

Example 3 - Longer 35-Day Cycle (From LMP Mode)

A woman with a 35-day cycle needs to find her adjusted due date and ovulation date.

LMP: March 1, 2026 | Cycle: 35 days

Ovulation = March 1 + (35 − 14) = March 1 + 21 = March 22, 2026

Fertile Window = March 17 to March 22, 2026

EDD = March 1 + 280 + (35 − 28) = March 1 + 287 = December 12, 2026

On May 17, 2026: 77 days from LMP → Week 11 exactly (First Trimester)

Conception: March 22, 2026 | EDD: December 12, 2026 | Week 11

Try this example →

Example 4 - Shorter 21-Day Cycle (From LMP Mode)

A woman with a shorter cycle calculates her earlier ovulation and adjusted due date.

LMP: April 15, 2026 | Cycle: 21 days

Ovulation = April 15 + (21 − 14) = April 15 + 7 = April 22, 2026

Fertile Window = April 17 to April 22, 2026

EDD = April 15 + 280 + (21 − 28) = April 15 + 273 = January 13, 2027

On May 17, 2026: 32 days from LMP → Week 4 + 4 days (First Trimester)

Conception: April 22, 2026 | EDD: January 13, 2027 | Fertile: April 17-22

Try this example →

❓ Frequently Asked Questions

How is the conception date calculated from a due date?+

Subtract 266 days (38 weeks) from the estimated due date. For example, an EDD of December 25, 2026 gives a conception date of approximately April 3, 2026. The 266-day figure represents the average fetal development period from fertilization to full-term birth. The LMP date estimate is EDD minus 280 days (40 weeks).

How is the conception date calculated from LMP?+

Ovulation (and likely conception) occurs on day: cycle length minus 14 before the next period. Formula: Conception Date = LMP + (Cycle Length - 14). For a 28-day cycle, conception is typically LMP + 14. For a 35-day cycle, it is LMP + 21. The due date is then LMP + 280 + (Cycle Length - 28).

What is the fertile window and when does it occur?+

The fertile window spans 6 days: the 5 days before ovulation plus the day of ovulation itself. Conception is possible throughout this window because sperm can survive up to 5 days in the reproductive tract. The two days immediately before ovulation and ovulation day itself have the highest conception probability (25-30% per day).

Can I know my exact conception date?+

Not exactly. Even with LMP and cycle data, the conception date is an estimate with a window of plus or minus a few days. Ovulation timing can shift due to stress, illness, travel, or hormonal changes. An early ultrasound (8-12 weeks gestation) is the most accurate way to establish gestational age and back-calculate a more precise conception date.

What is gestational age and how is it counted?+

Gestational age counts from the first day of the last menstrual period (LMP), not from conception. This is the standard medical measure. A 10-week gestational age means the pregnancy is 10 weeks from LMP, which is approximately 8 weeks from actual conception (since conception occurs around week 2 of gestational age).

What are the three trimesters and what weeks do they cover?+

The first trimester covers weeks 1-12: fertilization, implantation, embryo formation, and major organ development. The second trimester covers weeks 13-26: comfortable period with fetal movements beginning around week 20. The third trimester covers weeks 27-40: weight gain, lung maturation, and preparation for birth. Full-term is 37-42 weeks.

What if my due date from this calculator differs from my doctor's date?+

Small differences of 1-2 weeks are normal and usually reflect cycle length variations from the assumed 28 days or uncertainty about the exact ovulation date. Your doctor's due date, especially if set by a first-trimester ultrasound, is more reliable than LMP-based calculations and should be used for all medical planning.

How accurate is the conception date calculated from an EDD?+

The From Due Date mode assumes a standard 266-day conception-to-birth period. In practice, full-term births occur between 37 and 42 weeks, so the actual conception date may be plus or minus 2-3 weeks from this estimate. The result is best understood as the most likely conception window, not a fixed date.

Is this calculator suitable for IVF pregnancies?+

For IVF, the conception date is the egg retrieval or fertilization date. The EDD is typically the fertilization date plus 266 days. For a 5-day blastocyst transfer, the EDD is transfer date plus 261 days. Use the From Due Date mode with your IVF-assigned EDD to back-calculate the estimated conception date.

Why does the calculator show Week 2 for a very early pregnancy?+

Gestational age counts from LMP. At week 2 of gestational age, ovulation has not yet occurred and conception has not happened. This is technically a pre-conception week in medical counting but is labeled week 2 because the medical system counts from LMP. Actual fetal development begins at conception, which is around gestational week 2-3.

What does it mean if the calculator shows Days to EDD as negative?+

A negative days-to-EDD figure means the entered EDD is in the past. The calculator shows how many days ago the due date was. This is useful if you are looking up the conception date of a past pregnancy by entering a historical due date in the From Due Date mode.

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📌 Quick Tips

💡The most fertile days of your cycle are the day of ovulation and the two days immediately before it. Conception almost always occurs within this 3-day window, even though the fertile window spans 6 days.

💡Conception dates calculated from an LMP assume ovulation on day (cycle length minus 14). If your cycles are irregular, an early ultrasound at 8-12 weeks gives the most accurate conception date estimate.

💡A conceived embryo takes 6-10 days to implant in the uterine wall. A positive pregnancy test is typically detectable 10-14 days after conception, once hCG levels are high enough.

💡Sperm can survive in the female reproductive tract for up to 5 days. This means sex up to 5 days before ovulation can still result in conception on ovulation day.

💡The From Due Date mode works backwards from your EDD using the standard 266-day fetal development period from conception to birth, giving the most likely window of conception.