Conception Calculator

Enter your last menstrual period (LMP), ultrasound date, or IVF transfer date into the calculator above to estimate when you conceived and when your baby is due. Results appear instantly and cover your estimated due date, probable conception date, and how many weeks pregnant you are today.

A conception calculator works backward from known dates to pinpoint the most likely window when fertilization occurred. It also works forward to project your due date. Whether you know your last period date, have an ultrasound measurement, or went through IVF, the tool gives you a starting point for planning prenatal care and tracking your pregnancy.

Conception Date Calculator

Your conception date is the day the sperm fertilized the egg. Most women don't know the exact date of conception because fertilization happens inside the fallopian tube, out of sight, usually 12 to 24 hours after ovulation.

This conception date calculator estimates that date using the information you do have. If you enter your LMP and a typical cycle length, the tool assumes ovulation happened about 14 days before your next expected period. It then places conception near that ovulation window.

If you already know your due date from a healthcare provider, the calculator can work in reverse. It subtracts 266 days (38 weeks) from the due date to find your probable conception date. That is the standard interval from fertilization to delivery for a full-term pregnancy.

How to Calculate Your Conception Date

Calculating your conception date by hand takes three pieces of information:

1. The first day of your last menstrual period.
2. Your average cycle length (28 days is the default, but cycles range from 21 to 35 days or more).
3. A simple subtraction: cycle length minus 14 days equals your likely ovulation day.

For example, if your LMP was March 1 and your cycle is 30 days long, you likely ovulated around March 17. Conception probably happened on or very close to that date.

Sperm can survive in the reproductive tract for up to five days. The egg is viable for about 24 hours after ovulation. So your actual conception date could fall anywhere in a roughly six-day fertile window, even if you only had intercourse once during that time.

The calculator handles this math for you. Just enter your dates and cycle length.

Estimated Due Date From Your Last Period

The most common method for estimating a due date starts with your last menstrual period. The standard calculation, sometimes called Naegele's Rule, adds 280 days (40 weeks) to the first day of your LMP.

Here is how it works step by step:

1. Take the first day of your last period.
2. Add seven days.
3. Subtract three months.
4. Adjust the year if needed.

This formula assumes a 28-day cycle with ovulation on day 14. If your cycle is shorter or longer, the calculator adjusts automatically. A 35-day cycle, for instance, shifts ovulation later, which shifts the estimated due date later by the same number of days.

Your estimated due date is exactly that: an estimate. Only about 4% to 5% of babies arrive on their due date. Most are born within a two-week window on either side.

Due Date Calculator Based on Ultrasound Scan

An ultrasound scan measures the size of the embryo or fetus and compares it to standard growth charts. The measurement used in early pregnancy is called crown-rump length (CRL).

To use the ultrasound method in this calculator:

• Enter the date of your ultrasound.
• Enter the gestational age your provider measured at that scan (weeks and days).

The tool then calculates backward to estimate your LMP and forward to project your due date. Ultrasound dating is most accurate during the first trimester, roughly weeks 7 through 13. During that window, fetal biometry can estimate gestational age within about three to five days.

Later ultrasounds are less precise. By the second trimester, the margin of error widens to one to two weeks because babies start growing at different rates.

Conception Calculator for IVF Transfer Date

If you conceived through in vitro fertilization (IVF), the calculation changes because the date of embryo transfer is known precisely.

For a Day 3 embryo transfer, the calculator adds 263 days (or 37 weeks and 4 days) to the transfer date to estimate your due date. For a Day 5 blastocyst transfer, it adds 261 days (37 weeks and 2 days). The difference accounts for how far the embryo developed in the lab before transfer.

Your conception date in IVF is effectively the date of fertilization in the lab, which is typically the day of egg retrieval. The calculator can also convert your transfer date into an equivalent LMP so your gestational age matches standard pregnancy timelines used by your healthcare provider.

Enter your transfer date and embryo age (Day 3 or Day 5) in the calculator for an instant estimate.

When Did You Conceive? Gestational Age and Due Dates

Gestational age counts from the first day of your last menstrual period, not from the day you actually conceived. This means you are technically "two weeks pregnant" before fertilization even happens.

Fetal age (also called conceptional age) counts from the actual date of conception. It runs about two weeks behind gestational age.

This distinction matters because:

• Medical records, prenatal visits, and milestones use gestational age.
• The 40-week pregnancy timeline is measured in gestational weeks.
• When someone says they are 8 weeks pregnant, the embryo is actually about 6 weeks old.

So if you conceived two weeks ago, you are considered about 4 weeks pregnant by gestational age. That is not an error. It is how obstetric dating works.

The calculator displays both your gestational age and your estimated conception date so you can see both numbers clearly.

Can an Ultrasound Change Your Due Date?

Yes. An ultrasound can change your due date, and it commonly does. If the due date estimated from your LMP differs significantly from what the ultrasound measurement shows, your healthcare provider may adjust it.

The threshold for changing a due date depends on when the ultrasound is performed. Professional obstetric guidelines generally recommend redating when the discrepancy exceeds:

• 5 days in early pregnancy (before 9 weeks)
• 7 days between 9 and 13 weeks
• 10 to 14 days in the second trimester

Due Date Change in Early Pregnancy

First-trimester ultrasounds provide the most reliable dating. Embryos grow at a very consistent rate in the first 12 to 13 weeks, so a crown-rump length measurement gives a narrow margin of error.

If your early ultrasound shows a gestational age that differs from your LMP-based estimate by more than the threshold above, the ultrasound date typically becomes your official due date. This is especially common for women with irregular periods or uncertain LMP dates.

Once a due date is established by an early ultrasound, later scans usually do not change it again. Later measurements reflect individual growth variation rather than dating error.

How to Determine Your Due Date if You Have Irregular Periods

Standard due date formulas assume a regular cycle, usually 28 days. If your periods are irregular, the LMP-based calculation may be unreliable.

Options that work better for irregular cycles:

• Ultrasound dating. A first-trimester ultrasound scan is the most accurate alternative. It measures the embryo directly and does not depend on cycle length.
• Ovulation tracking. If you tracked ovulation with basal body temperature, ovulation predictor kits, or cervical mucus monitoring, enter that known ovulation date. The calculator adds 266 days to estimate your due date.
• Known intercourse date. If you know the only possible date of conception, the tool can estimate from there.

If you are pregnant but unsure when you conceived because your periods are unpredictable, an early ultrasound is the single best way to determine your due date. Talk with your healthcare provider about scheduling one.

Pregnancy Due Date Calculator vs. Conception Date Calculator

These two tools answer different questions using the same underlying math.

This calculator does both. Enter the data you have, and it returns your estimated due date, your likely conception date, and how many weeks pregnant you are.

If you already know your due date from a prenatal visit, enter it to find your estimated conception date. If you only know your last period date, the tool projects forward to your due date and backward to your conception window.

When to Get Pregnant: Ovulation and Fertilization

Your fertile window is the span of days each cycle when pregnancy is possible. It typically covers the five days before ovulation plus ovulation day itself.

Here is why:

• Sperm can live in the reproductive tract for up to five days.
• The egg survives about 12 to 24 hours after release from the ovary.
• Fertilization happens when sperm meets egg in the fallopian tube.

Most women ovulate about 14 days before their next period starts. For a 28-day cycle, that is around day 14. For a 32-day cycle, ovulation is closer to day 18.

Tracking ovulation improves timing. Common methods include:

• Ovulation predictor kits (detect the LH surge 24 to 36 hours before ovulation)
• Basal body temperature charting (shows a small rise after ovulation)
• Cervical mucus observation (stretchy, clear mucus signals peak fertility)

Conception does not always happen on the first try. Many healthy couples take several months. Timing intercourse during the fertile window gives the best chance each cycle.

Estimate Your Due Dates: What 280 Days and 40 Weeks Mean

A full-term pregnancy lasts about 280 days, or 40 weeks, counted from the first day of the last menstrual period. That equals roughly 266 days (38 weeks) from the actual date of conception.

These numbers represent an average, not a deadline. A healthy delivery can happen anywhere from 37 to 42 weeks. The 40-week mark simply anchors the pregnancy timeline.

Here is a quick breakdown of the trimesters within that 280-day window:

• First trimester: Weeks 1 through 13 (conception through early fetal development)
• Second trimester: Weeks 14 through 27
• Third trimester: Weeks 28 through 40

Your estimated due date gives your healthcare provider a reference point for scheduling prenatal care, growth checks, and lab work. It also helps you plan practically for maternity leave, nursery setup, and birth preparation.

Remember that "due date" means "estimated date." Your baby will arrive on its own schedule.

Free Pregnancy Calculator Limitations

This calculator uses standard obstetric formulas and the dates you provide. It gives a reasonable estimate, but several factors can affect accuracy:

• Irregular or uncertain period dates reduce LMP-based precision.
• Cycle lengths outside the typical range shift ovulation timing.
• Late ovulation or delayed implantation can alter actual conception timing.
• Individual variation in fetal growth affects ultrasound-based estimates.

The calculator cannot account for medical conditions, multiple pregnancies, or unique circumstances that influence gestational timing.

This Calculator Provides an Estimate, Not a Diagnosis

This tool is for informational and planning purposes only. It does not replace a clinical assessment by a qualified healthcare provider.

Your doctor or midwife will confirm your due date using your medical history, physical examination, and ultrasound findings. If you have concerns about your pregnancy dates, gestational age, or prenatal care, schedule a visit with your healthcare provider.

Use this free pregnancy calculator as a helpful starting point. Rely on your prenatal care team for medical decisions.