Due Date by Category: How Each Type Is Set

This calculator works out an estimated due date based on the category you choose—last menstrual period (LMP), conception date, or embryo transfer date—and returns the expected delivery date along with the current gestational age. You enter the relevant date and cycle length (for LMP) or the transfer day (for IVF), and you get the due date plus how far along you are. The result is an estimate: only about 4% of babies arrive on their exact due date, and most are born within two weeks either side.

Formula

Each category uses a different starting point, but all rely on the same average pregnancy length of 280 days (40 weeks) from the first day of the last menstrual period, or 266 days (38 weeks) from conception.

• LMP method: Due date = LMP date + 280 days + (cycle length − 28 days). The cycle adjustment accounts for a cycle longer or shorter than the standard 28 days.
• Conception method: Due date = conception date + 266 days. This assumes ovulation and fertilization occurred on the conception date.
• Embryo transfer method: Due date = transfer date + (266 − transfer day). For a day-5 blastocyst transfer, subtract 5 days from 266; for a day-3 embryo, subtract 3.

All variables are dates in the format YYYY-MM-DD. Gestational age is calculated as (today − LMP date) / 7, expressed in weeks.

Worked Example

Using the default values from the calculator:

1. LMP date: 2024-01-15, cycle length 28 days. Due date = 2024-01-15 + 280 days = 2024-10-21.
2. Conception date: 2024-01-29. Due date = 2024-01-29 + 266 days = 2024-10-21.
3. Embryo transfer: 2024-02-05, day-5 transfer. Due date = 2024-02-05 + (266 − 5) = 2024-02-05 + 261 days = 2024-10-23.
4. Gestational age: If today is 2024-07-01, then (2024-07-01 − 2024-01-15) = 168 days / 7 = 24.0 weeks.

The calculator gives the same results: due dates of 2024-10-21, 2024-10-21, and 2024-10-23, and a gestational age of 24.0 weeks.

Why the Due Date Changes by Category

The three categories exist because not everyone knows their conception date, and assisted reproduction provides a precise starting point. The LMP method is the most common because it only requires remembering the first day of your last period. However, it assumes a regular 28-day cycle with ovulation on day 14. If your cycle is longer or shorter, the due date shifts accordingly—that’s why the calculator includes a cycle length input.

Conception dates are known when someone tracks ovulation with basal body temperature, ovulation predictor kits, or fertility apps. The due date is simply 266 days from that date, which is the average time from fertilization to birth.

Embryo transfer dates are known with certainty in IVF. The transfer day matters because a day-5 embryo has already developed for five days, so the remaining pregnancy is shorter. The formula subtracts the transfer day from 266 to account for that pre-transfer development.

How to Choose the Right Category

Pick the category that matches the information you have:

• LMP: Use this if you have a regular cycle and know the first day of your last period. It’s the standard method used by most healthcare providers.
• Conception: Use this if you know the exact date of ovulation or intercourse that led to conception. This is common for those who track fertility signs.
• Embryo transfer: Use this if you had IVF with a known transfer date. You need to know whether the embryo was transferred on day 3 or day 5.

If you have irregular cycles, the LMP method becomes less accurate. In that case, an ultrasound in the first trimester is the most reliable way to date a pregnancy, because it measures the embryo’s size rather than relying on menstrual history.

Common Mistakes and Misconceptions

A frequent mistake is adding 280 days to the conception date, which overestimates the due date by about two weeks. Remember: 280 days is from the LMP, not from conception. Another error is forgetting to adjust for cycle length—if your cycle is 35 days, ovulation occurs around day 21, so the due date is a week later than the standard 280-day calculation.

Many people believe the due date is a deadline, but it’s really an estimate. Only about 4% of babies are born on their exact due date. A normal pregnancy can last anywhere from 37 to 42 weeks, and due dates are used to schedule tests and monitor progress, not to predict the exact day of birth.

Also, the due date does not change if you have a short luteal phase (the time after ovulation). The luteal phase is relatively constant at about 14 days, so cycle length variations come from the follicular phase before ovulation.

Limitations of the Formula

These formulas are based on averages and assume a regular cycle, ovulation on day 14, and a standard 280-day pregnancy. Real pregnancies vary: first-time mothers tend to go slightly longer, and multiples (twins, triplets) are often born earlier. The due date from an early ultrasound can differ from the LMP-based date by several days, and healthcare providers usually trust the ultrasound if the discrepancy is more than a week.

The calculator does not account for individual medical history, maternal age, or other factors that might influence pregnancy length. It is an educational tool, not a medical device. Always confirm your due date with a healthcare provider, especially if you have irregular cycles or used assisted reproduction.

Frequently Asked Questions

What is the most accurate way to calculate a due date?

An ultrasound measurement of the embryo in the first trimester (around 8–12 weeks) is the most accurate method. It uses the crown-rump length to estimate gestational age, and it is accurate to within about 5–7 days. LMP-based dates are only accurate if your cycle is regular and you are sure of the date.

Why is my due date different from the one my doctor gave me?

Doctors may use an ultrasound or a pregnancy wheel that assumes a 28-day cycle. If your cycle is longer or shorter, or if the ultrasound measurement differs from your LMP date, the due date can shift. The calculator lets you adjust for cycle length, which may explain a discrepancy.

Can I calculate the due date from the date of intercourse?

Not reliably, because sperm can survive for up to five days in the reproductive tract, so intercourse can occur several days before ovulation. The conception date is the day of ovulation/fertilization, not necessarily the day you had sex. If you know your ovulation date, use the conception method.

How does a day-3 transfer differ from a day-5 transfer?

A day-3 embryo is transferred three days after fertilization, while a day-5 blastocyst is transferred five days after. Because the day-5 embryo has already developed for two extra days, the remaining pregnancy is two days shorter. The formula subtracts the transfer day from 266 to account for this.

What if I don’t know my LMP or conception date?

If you have no idea, an ultrasound is the best option. Without any dates, the calculator can’t help. If you know the approximate week of conception, you can use that as a rough estimate, but it will be less accurate.

Is a 40-week pregnancy counted from LMP or conception?

From LMP. The 280 days (40 weeks) is counted from the first day of the last menstrual period. From conception, it’s 266 days (38 weeks). The two-week difference is the time between the LMP and ovulation.

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in d | LMP date | date | 2024-01-15 | 0
in n | Cycle length | days | 28 | 0
in t | Conception date | date | 2024-01-29 | 0
in e | Embryo transfer date | date | 2024-02-05 | 0
in x | Transfer day (3 or 5) | days | 5 | 0
out due_lmp | Due date (LMP) | date | d + 280 + (n - 28) | 0
out due_conception | Due date (conception) | date | t + 266 | 0
out due_transfer | Due date (embryo transfer) | date | e + (266 - x) | 0
out ga | Gestational age (weeks) | weeks | (today - d) / 7 | 1