To calculate your due date, count 280 days from the first day of your last menstrual period, which is the same as adding 7 days and subtracting 3 months (Naegele’s rule). If you know your conception date, use it as a starting point for an estimate. Either way, treat the result as an estimate: an early ultrasound and your clinician’s obstetric estimate can move the date.
TL;DR:
- For cycles that differ from 28 days, Naegele’s rule can miss ovulation timing because it assumes ovulation on day 14; conception dating adds 266 days.
- Before 9 weeks, a difference over 5 days can prompt redating; between 9 and 13 weeks, the threshold is more than 7 days.
- If your last period is unknown or unreliable, ask for an early dating ultrasound, ideally before 13 weeks, and bring fertility records and known dates.
- A first trimester crown rump length scan dates pregnancy within about 5 to 7 days, while later scans provide less precise estimates.
- Delivery should not be scheduled before 39 weeks unless there is a medical reason, so ask your clinician why an earlier induction is recommended.
Table of Contents
- How do you work out your due date step by step?
- Why does ultrasound often change the due date?
- What if you don’t know your last period or your cycles are irregular?
- How accurate are due date calculators?
- Can you practice with two worked examples?
- What I’d do first if I were you
- How can our planner help you track your dates?
- FAQ
- Sources
How do you work out your due date step by step?
Most due date math starts from one assumption: a typical pregnancy lasts 40 weeks, or 280 days, counted from the first day of your last normal period. Naegele’s rule is the shortcut. You add 7 days to that first day, then subtract three months.
The rule assumes a 28-day cycle with ovulation on day 14, which is a simplification that real cycles often break. If your cycles run shorter or longer, ovulation likely happened earlier or later than the formula expects, and your date may shift accordingly.
Here is how the main approaches compare:
- LMP method: Best when your cycles are regular and you remember the first day of your last period clearly.
- Conception method: Best when you know the day you conceived, for example from ovulation tracking or a single encounter. Add 266 days, or 38 weeks.
- Online calculators: They run the same math, using your period date, cycle length or conception date. They save you the arithmetic but do not add accuracy.
- Assisted reproduction: If you had IVF or another fertility treatment, your clinic uses the known fertilization or transfer date, not a period date.
To do the LMP math by hand:
- Write down the first day of your last period (the first day of full flow, not spotting).
- Add 7 days to that date.
- Subtract 3 months from the result.
- Adjust a few days earlier or later if your cycle is clearly shorter or longer than 28 days.
Why does ultrasound often change the due date?
Your calculated date is a starting point. ACOG states that first-trimester ultrasound is the most accurate method for establishing or confirming gestational age. In early pregnancy, the sonographer measures the crown–rump length (CRL), the distance from the top of the embryo’s head to its bottom. Averaging several CRL measurements gives the best dating accuracy early in the first trimester.
ACOG also sets clear thresholds for when the ultrasound date replaces your period-based date:
- Ultrasound before 9 0/7 weeks: the date is revised if it differs from your LMP date by more than 5 days.
- Ultrasound between 9 0/7 and 13 6/7 weeks: the date is revised if the difference is more than 7 days.
- No ultrasound before 22 0/7 weeks: the pregnancy is considered suboptimally dated.
Why this happens: Only about half of women accurately recall their LMP, and ovulation rarely lands exactly on day 14.
Once your clinician settles on a date, it becomes your obstetric estimate (OE). In the U.S., the OE is the clinician’s best estimate of gestation in completed weeks, and it is the figure used in medical and government records. A revised date is not a sign that something is wrong. It simply means your care team has a more reliable timeline for scheduling screenings and tracking growth.
What if you don’t know your last period or your cycles are irregular?
This is common, and clinicians have a clear plan for it. When your LMP is unknown or unreliable, the earliest ultrasound, ideally before 13 6/7 weeks, is used for dating. Here is what you can do in the meantime:
- Estimate a possible date range, even a rough one, based on when you first noticed symptoms or had a positive test.
- Call your clinician early and ask about scheduling an early dating ultrasound.
- Bring fertility or treatment records if you conceived with assisted reproduction.
- Note any bleeding, the date of your first positive test, and any ovulation tracking you did.
Pro Tip: Keep every date in one place, a planner or a notes app on your phone, so you can hand your clinician a clean timeline at your first visit.
If you want to understand your fertile window better for future cycles, our free ovulation calculator is one tool we offer for conception planning. For this pregnancy, though, your clinician’s ultrasound will be the date that counts.
How accurate are due date calculators?
Calculators are only as good as the date you type in. A first-trimester CRL measurement is accurate to about plus or minus 5 to 7 days, and that precision gets wider in the second trimester and is weakest in the third. A calculator that relies on an LMP you half remember cannot beat that.
Worth knowing: Pregnancy is conventionally measured as 40 weeks from your LMP, which is 38 weeks from conception.
Here is how to think about your due date:
- It is a planning date, not a prediction of the day your baby will arrive.
- Full-term births commonly fall in a window around the due date rather than on it.
- Your date may change after an ultrasound, and that is normal.
- CDC guidance says delivery should not be scheduled before 39 weeks unless there is a medical reason, because important growth continues in the final weeks.
So expect a window and stay flexible. If your clinician recommends induction or a scheduled delivery, ask whether the reason is medical.
Can you practice with two worked examples?
Here are two quick ones. The dates are illustrative.
- Naegele’s rule: Say your last period began on October 10. Add 7 days to get October 17. Subtract 3 months to get July 17. Your estimated due date is July 17. If an early ultrasound measures more than 5 to 7 days apart from that, your clinician will likely use the ultrasound date instead.
- Conception date: Say you conceived on November 1. Add 38 weeks (266 days) to land on July 25. For an embryo transfer, the same idea applies using the embryo’s age and the transfer date. Early ultrasound can still adjust the result.
What I’d do first if I were you
The most useful thing you can do is not to find the perfect calculator. It is to get an early ultrasound on the calendar and write down the dates you already know. Most online advice stops at the formula, as if the math were the hard part. It isn’t. The hard part is that the inputs, a remembered period date or an assumed ovulation day, are often shaky, and no formula can fix that.
I’d also loosen my grip on the single date. A due date is a midpoint in a window, and families who plan around a window tend to feel calmer when the date shifts. If you are making plans for work leave, travel or help at home, build in a couple of weeks of flexibility on both sides.
Finally, track things as they happen. Write down the first day of each period, the date of your positive test, and every appointment. That habit can help make every visit more organized.
— Rebeka
How can our planner help you track your dates?
We built the Baby Bare Essentials pregnancy planner as a calm place to keep your records alongside your clinician’s care. It is an organizing tool, not medical advice. Inside, you can:
- Keep your pregnancy records organized
- Keep your planning notes in one spot
- Keep your planning materials together
You can explore the Physical Planner and Digital Edition whenever you’re ready, and learn more about safe cleaning during pregnancy to protect your health as you prepare for your baby.
This article is general information, not a substitute for advice from a qualified doctor. Consult a qualified healthcare professional about your own circumstances before acting on anything here.
FAQ
How accurate are due date calculators?
Calculators are reliable math but rely on the date you enter. A first-trimester ultrasound is accurate to about 5 to 7 days, so your clinician’s ultrasound date is usually the more dependable one.
How do you count 280 days of pregnancy?
Start on the first day of your last period and count forward 280 days, which equals 40 weeks. A shortcut is to add 7 days to that date and subtract 3 months, which is Naegele’s rule.
Can your due date change after an ultrasound?
Yes. If an early ultrasound differs from your LMP-based date by more than the ACOG thresholds, your clinician will usually adopt the ultrasound date and record it as your obstetric estimate.
When is my baby actually likely to arrive?
Most babies arrive within a window around the due date, not on it. Plan for a range of a couple of weeks, and ask your clinician about timing if you have concerns.
Sources
We write this guide using clinical sources. We lean on ACOG for the ultrasound thresholds, on CDC reporting standards for how the obstetric estimate is recorded, and on CDC guidance for timing of delivery. Your clinician has the final word on your date. Keeping your own records (LMP, test dates, ultrasound results) makes those conversations clearer and faster.
Leave a Reply