How to Calculate Your Due Date (Last Period vs IVF)

The moment you see two lines on a pregnancy test, the first question is almost always the same: when is the baby due? The answer is a single date that organizes the next nine months — appointments, ultrasounds, maternity leave, and the nursery countdown. But here's the part nobody tells you up front: your due date is an estimate, not an appointment. It marks 40 weeks of pregnancy, yet only about 1 in 20 babies actually arrives on that exact day.

In this guide we'll walk through both ways to calculate a due date — the classic last-menstrual-period method used for natural conception, and the more precise IVF dating used after fertility treatment. We'll work real examples, explain the difference between gestational and fetal age, and show you why an early ultrasound usually beats the math.

What Your Due Date Actually Means

A full-term pregnancy is counted as 280 days, or 40 weeks, measured from the first day of your last menstrual period (LMP). That's the convention obstetricians use worldwide. The clinical name for the due date is the EDD — Estimated Date of Delivery (sometimes EDC, Estimated Date of Confinement).

The thing that surprises most first-time parents is that those 40 weeks are counted from a day when you weren't even pregnant yet. Because pinpointing the exact moment of conception is nearly impossible, the medical world standardized on the LMP — a date you can usually remember. Conception typically happens about two weeks after the LMP, which means the baby is only developing for roughly 38 weeks even though you're "40 weeks pregnant." More on that distinction below.

Term itself is a range, not a single day. Babies born between 37 and 42 weeks are considered to have arrived at term. The due date simply marks the midpoint of the window where birth is most likely — it is a target, not a guarantee.

Naegele's Rule: Calculating From Your Last Period

For a pregnancy conceived naturally, the standard method is Naegele's rule, named after the 19th-century German obstetrician Franz Naegele. The classic version is a quick mental shortcut:

EDD = (First day of LMP) − 3 months + 7 days + 1 year

Mathematically, that's the same as simply adding 280 days to the first day of your last period. Both produce the same answer; the "minus 3 months, plus 7 days" trick is just easier to do in your head.

Here's what each piece means:

• LMP — the first day of your last menstrual period, not the last day.
• +280 days — the assumed length of a full-term pregnancy from that date.
• Cycle-length adjustment — the standard rule assumes a textbook 28-day cycle. If yours runs longer or shorter, the estimate shifts (we cover this next).

The most common mistake is using the wrong starting point. Naegele's rule keys off the first day you started bleeding, not the day your period ended and not the day you think you conceived. Get that date right and the rest is arithmetic.

Adjusting for a Longer or Shorter Cycle

Naegele's rule bakes in an assumption: a 28-day cycle with ovulation on day 14. Plenty of people don't fit that mold. If your cycles are consistently longer or shorter, your ovulation — and therefore conception — happens earlier or later than the textbook assumes, and your due date should move with it.

The adjustment is simple. Take the difference between your average cycle length and 28 days, and add it to (or subtract it from) the standard estimate:

• Longer cycles (say, 32 days): you ovulate later, so add 4 days. Your due date moves later.
• Shorter cycles (say, 25 days): you ovulate earlier, so subtract 3 days. Your due date moves earlier.

So a person with a steady 32-day cycle would calculate LMP + 280 days, then add another 4 days — landing on LMP + 284 days. Our Due Date Calculator does this adjustment for you automatically: enter your LMP and your typical cycle length, and it shifts the estimate accordingly so you're not stuck assuming a 28-day cycle that isn't yours.

A Fully Worked LMP Example

Let's run the numbers for someone whose last period started on January 10, 2025, with a standard 28-day cycle.

Both methods land on the same date: October 17, 2025. Now suppose this person actually has a 31-day cycle instead of 28. We add 3 days, moving the estimate to October 20, 2025. That three-day shift may seem trivial, but near the end of pregnancy it can be the difference between being told you're "overdue" and being right on schedule.

How IVF Due Dates Are Different (and More Precise)

If you conceived through in vitro fertilization, you have something most pregnancies don't: a known conception date. The embryo's age is tracked precisely in the lab, so there's no need to estimate ovulation from a menstrual cycle. That makes IVF due dates noticeably more accurate.

The math depends on which day your embryo was transferred. Embryologists count the age of the embryo from the day of egg retrieval and fertilization ("Day 0"), and the transfer usually happens on either Day 3 (a cleavage-stage embryo) or Day 5 (a blastocyst). The key number to anchor on is that a full-term pregnancy is 266 days from fertilization — that's the same as 280 days from the LMP, because conception happens about 14 days after the LMP (266 + 14 = 280). Since the embryo has already lived for a few days in the lab, you simply subtract its age at transfer from 266:

• Day 3 transfer: EDD = transfer date + 263 days (266 − 3, because the embryo is already 3 days old at transfer).
• Day 5 transfer (blastocyst): EDD = transfer date + 261 days (266 − 5, because a blastocyst is 5 days old at transfer).
• Frozen embryo transfer (FET): use the same Day 3 or Day 5 rule based on the embryo's age when it was frozen — the time spent frozen doesn't count toward gestation, so a thawed Day 5 blastocyst still uses +261 days.

Here's a quick comparison for a transfer performed on March 1, 2025:

Because the conception date is exact, an IVF due date doesn't carry the cycle-length guesswork that LMP dating does. Our IVF Due Date Calculator lets you pick your transfer type — Day 3, Day 5, or a frozen transfer — and returns the EDD without you having to memorize the 263-vs-261 offsets.

Gestational Age vs. Fetal Age

One of the most confusing parts of early pregnancy is hearing two different "ages" for the same baby. They are not contradictory — they just count from different starting points.

• Gestational age is counted from the first day of your LMP. This is the number your doctor uses, the number on your ultrasound report, and the basis of the "40 weeks" framework. At conception, you are already considered 2 weeks gestational.
• Fetal age (also called conceptional or developmental age) is counted from actual conception. It's always about 2 weeks less than gestational age.

So when an app says you're "8 weeks pregnant," the embryo has only been developing for about 6 weeks. Both numbers are correct; they're just answering different questions. Clinically, gestational age is the standard because it's tied to the LMP, which is far easier to date than the precise moment of conception. The one big exception is IVF, where fetal age is known exactly — which is why IVF dating is so reliable.

The Three Trimesters at a Glance

Pregnancy is divided into three trimesters, each marking a distinct phase of development. The boundaries are based on gestational age:

• First trimester — weeks 1 to 13. Conception, implantation, and the formation of all major organs. This is when early-pregnancy symptoms are usually strongest and when the most important dating ultrasound happens.
• Second trimester — weeks 14 to 27. Often the most comfortable stretch. The anatomy scan around weeks 18–20 checks development and can confirm the baby's sex.
• Third trimester — weeks 28 to 40+. Rapid growth and final maturation as the baby prepares for birth. Term begins at week 37.

Want to know exactly how many days along you are today, or how old the baby will be on a given date? Our Age Calculator is handy for counting the precise number of days between any two dates — useful for tracking milestones from your LMP, transfer date, or birth day.

Why an Early Ultrasound Beats the Math

For all the elegance of Naegele's rule, the single most accurate way to date a pregnancy is an early ultrasound. In the first trimester — ideally between weeks 8 and 13 — embryos grow at a remarkably consistent rate across all pregnancies. A sonographer measures the crown-rump length (the distance from the top of the head to the bottom of the spine) and can pin down gestational age to within about 3 to 5 days.

That precision matters because LMP dating breaks down whenever the assumptions don't hold: irregular cycles, uncertain memory of the last period, late ovulation, or spotting mistaken for a period. If an early ultrasound disagrees with the LMP estimate by more than about a week, doctors typically re-date the pregnancy using the ultrasound. As pregnancy advances, ultrasound dating becomes less precise — by the third trimester, genetics and growth differences make estimates vary by weeks — which is exactly why the early scan is the gold standard.

Why Only 1 in 20 Babies Arrives on the Due Date

Here's the reality check every expecting parent should internalize: only about 5% of babies are born on their estimated due date. Roughly 1 in 20. That isn't a flaw in the calculation — it's biology. Normal pregnancy length varies naturally from person to person, and even from pregnancy to pregnancy in the same person.

The vast majority of births cluster in the window from a couple of weeks before to a couple of weeks after the EDD. Babies arriving anywhere between 37 and 42 weeks are considered to have arrived right on time. First pregnancies tend to run slightly longer on average, and factors like genetics and previous birth history all nudge the actual day around.

The practical takeaway: treat your due date as the center of a range, not a deadline. Build flexibility into maternity leave plans, hospital bags, and childcare arrangements. The number is genuinely useful for planning and for tracking your baby's development week by week — just don't circle it in permanent marker.

The Bottom Line

Calculating a due date comes down to one core idea: 40 weeks (280 days) from the first day of your last period, adjusted for your cycle length. If you conceived through IVF, you can be more precise — add 263 days to a Day 3 transfer or 261 days to a Day 5 blastocyst transfer. Either way, an early ultrasound is the most reliable confirmation, and the final date is always an estimate, since only about 5% of babies arrive exactly on schedule. Use our Due Date Calculator or IVF Due Date Calculator to get your EDD in seconds, then enjoy the countdown. This article is for informational purposes only and is not medical advice; always consult your obstetrician or midwife for guidance specific to your pregnancy.