How accurate is a due date?

It's an estimate. Only around 4% of babies are born on their due date; most arrive between 37 and 42 weeks.

My cycles are longer than 28 days. Does that matter?

Yes. Longer cycles usually mean later ovulation, so the calculator adds the extra days to your due date. Shorter cycles move it earlier.

Why do IVF due dates differ for day-3 and day-5 embryos?

A day-5 embryo (blastocyst) is two days further developed at transfer than a day-3 embryo, so the due date comes two days sooner from the transfer date.

My doctor gave me a different due date. Which is right?

Follow your care provider. A first-trimester ultrasound measurement often refines the date, and that's the one used for your care.

Due Date Calculator

Estimate your baby's due date from your last period, conception date, IVF transfer or an ultrasound, and see how far along you are today.

Informational only, not medical advice. Talk to a doctor.

First day of last period

Average cycle length

days

Enter valid values to see results.

How it works

Pick the method that matches what you know. Last menstrual period (LMP) is the most common; an early ultrasound is usually the most accurate.

Pregnancy is counted as 280 days (40 weeks) from the first day of the last period. Other methods are converted to that same timeline.

We then show your gestational age today, the trimester, days remaining and key milestones such as the anatomy scan window.

Formula

LMP: due = LMP + 280 + (cycle − 28) Conception: due = date + 266 IVF: due = transfer + 261 (day 5) or + 263 (day 3) Ultrasound: due = exam + (280 − gestational days)

Where:

LMP: = First day of your last menstrual period
cycle: = Average cycle length in days
gestational days: = Weeks × 7 + days measured at the scan

Worked example

Your last period started January 1, 2026, and your cycle is 28 days.

Due date = January 1 + 280 days = October 8, 2026. A conception date of January 15 gives the same result: January 15 + 266 days.

Understanding Your Estimated Due Date

Recognizing that a due date is a helpful estimate rather than a fixed appointment is vital for managing expectations during the third trimester. Most births occur within a two-week window around the calculated date, so staying flexible helps reduce anxiety if your baby arrives slightly earlier or later than anticipated. Your medical team will monitor fetal development and placenta health to ensure the timing remains safe as you approach the end of your pregnancy journey.

Tracking your menstrual cycle accurately before conception can improve the precision of initial estimates. If your cycles are irregular or vary significantly in length, a dating scan performed during the first twelve weeks typically provides the most reliable timeline. Healthcare providers often adjust the due date based on these early ultrasound measurements because embryos grow at a very consistent rate during the initial stage of biological development.

Practical Tips and Common Errors

A frequent mistake is assuming that pregnancy lasts exactly nine months; in reality, a full term is forty weeks, which is closer to ten months from the start of your last period. Always note the first day of your cycle rather than when you believe you conceived, as most medical professionals use the former for standardization. Additionally, ensure you communicate any history of IVF or known ovulation dates to your doctor, as these specific details can refine the calculation significantly.

Remember that lifestyle factors and maternal health can influence when labor begins naturally. While this tool provides a statistical baseline, your individual experience will depend on your body's unique readiness. Stay in close contact with your obstetrician or midwife as you enter the final month to prepare for the arrival of your new family member.

Frequently asked questions

The first day of the last period is a date most people know, while the exact day of conception usually isn't. By convention, week 1 starts at LMP, about two weeks before conception.

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For informational purposes only. This calculator does not provide medical advice. Consult a qualified professional before making decisions based on these results.