Due Date Is Naegele’s Rule, Not an Ultrasound

DevOkk’s Due Date Calculator implements that rule, with an optional cycle-length shift, in the browser. Default example: LMP 2026-01-01, 28-day cycle → EDD 2026-10-08. That is date arithmetic. It is not medical advice. It is not a diagnosis of pregnancy. It is not a substitute for prenatal care.

BMI and weight tools are a different mistake (treating a ratio as a disease). That split is Why BMI is not a diagnosis. Do not mix a due date with a “healthy weight for pregnancy” from an adult non-pregnant BMI band. Those pages are not gestational charts.

What Naegele’s rule is counting

The 280 days is 40 weeks from LMP, which already includes about two weeks before a textbook ovulation. Gestational age in clinic is often spoken in weeks from LMP, not from fertilization. That is why “8 weeks pregnant” does not mean the embryo has existed for eight weeks. A web calculator that adds 280 days is staying inside that same convention.

Conception-date gadgets that subtract 266 days or “14 days after LMP” are still models. Sperm and oocyte timing, luteal length, and irregular ovulation are not in the form. DevOkk does not ship a conception-timestamp product. Do not use EDD arithmetic to prove when sex happened.

Cycle length is a lever, not a lab

If the cycle is usually 35 days rather than 28, ovulation may be later, and some teaching adjustments add the extra days to the EDD. The optional cycle field exists for that classroom move. It still assumes a regular cycle you can describe with one integer.

Irregular periods, PCOS, recent pregnancy, breastfeeding, and coming off hormonal contraception make “first day of last period” a noisy zero. Implant bleeding is not always a period. If LMP is unknown, Naegele cannot be rescued by a prettier default date.

280 days from LMP is not 266 days from fertilization

A textbook 28-day cycle puts ovulation around day 14. Fertilization dating is often taught as about 266 days (38 weeks) from that event. Naegele’s 280 days from LMP already includes those notional two weeks. If you subtract 266 from today and also add 280 to LMP, you are mixing clocks. Pick LMP gestational age or a known fertilization/transfer date, not both.

“First day of last period” means the first day of bleeding that counts as a period in that cycle, as you would tell a midwife. Spotting after a positive test is not automatically a new LMP. Cycle apps that auto-shift LMP from a logged flow can disagree with what you type on DevOkk. The calculator believes the date in the field.

Leap days fall inside a 280-day span sometimes. Date libraries handle that; humans counting “nine months plus seven days” on a paper calendar sometimes do not. Trust the ISO date add, not a rhyme, if you need the classroom EDD.

Why ultrasound dating exists

Crown–rump length in the first trimester is used because embryos of the same gestational age are similar in size, while LMP recall is not. When LMP and early ultrasound disagree beyond the clinic’s threshold, teams often re-date from the scan. A browser result that still shows the LMP-only date is not a veto. Bring the scan to the appointment; do not bring a screenshot of October 8 as a counter-argument.

Later ultrasounds estimate size, not a perfect clock. Growth restriction and large-for-gestational-age fetuses are why third-trimester size is a poor dating method. Do not “update” an EDD every month from a random online growth chart.

IVF, IUI, and known transfer dates

If fertilization or transfer is known from a clinic, dating follows embryo age and transfer day, not LMP + 280. Using Naegele on a stimulated cycle’s last bleed can be the wrong zero. This article will not walk through every ART protocol. The rule is: clinic dating wins. The DevOkk page is for the LMP teaching case.

Twins, late ovulation, and “overdue”

Multiple pregnancy dating still starts from LMP or scan rules the clinic names; Naegele does not become more accurate because there are two fetuses. Late ovulation in a long cycle is exactly what the cycle-length adjustment tries to gesture at, and still fails if the cycle length is a guess.

“Overdue” in common speech means past the EDD. Clinically, term is a window of weeks. Induction discussions use the obstetric record, cervical status, and guidelines - not a tools-site reprint of October 8. Do not walk into triage with a browser EDD as the only clock. If the clinic re-dated last Tuesday, their EDD is the one on the record, even if Naegele still prints October 8.

A positive home test also does not set gestational age. hCG timing varies. The test date is not LMP. A dating app that starts the clock from a positive test will disagree with Naegele from LMP; both can be internally consistent and still not match a first-trimester CRL.

What a due date is not promising

Most births are not on the EDD. A term window of weeks is the clinical reality. Inducing or waiting is a medical decision. A calculator that prints one day is a midpoint label, not an appointment with the baby.

Complications, bleeding, pain, and reduced fetal movement are emergency or clinic problems. They are not solved by recomputing Naegele.

What the sample dates are for

LMP 2026-01-01 and EDD 2026-10-08 exist so you can check the add: 31+28+31+30+31+30+31+31+8 = 280 days through 8 October, accounting for 2026 not being a leap year. If your local calendar widget disagrees, the widget is wrong or you used a 28-day “month” rhyme. The tool is not “biased toward October.” It added 280.

If you enter today’s date as LMP, you get an EDD about forty weeks out. That is still not proof of pregnancy. It is date math on a field you filled. Do not screenshot the sample January 1 / October 8 pair and treat it as your obstetric chart.

Privacy: a date can still be identifying

The addition runs in JavaScript. The LMP is not sent to DevOkk to finish the estimate. localStorage can remember the last LMP on that browser. On a shared computer, that is a privacy incident waiting to happen. Clear it. Analytics still load. Local does not mean “this date is only yours if someone else uses the laptop.”

Do not enter someone else’s LMP to gossip about a due date. That is not a demo.

Naegele is a mnemonic, not a fertility workup

The “add seven days and nine months” rhyme is a paper-calendar version of 280 days and fails at month lengths. January 1 + 9 months + 7 days is a folk path to October 8 in a non-leap story; use the 280-day add. The rule assumes a cycle you can describe. It does not diagnose pregnancy, viability, or ectopic risk. A missed period plus a calculator is not prenatal care.

Implantation bleeding confused with LMP is a common way to be off by a week before any scan. If LMP is a guess, say it is a guess. The field will still add 280 days to whatever date you typed.

Unknown LMP: the tool cannot invent a date. Clinic dating from exam and ultrasound is the path. Do not pick January 1 because it is the sample.

Trimester math from the same clock

People ask the calculator for “how many weeks am I.” That is (today − LMP) in days, divided by 7, under the same LMP convention - still not a scan. DevOkk’s primary output is the EDD. If you count weeks yourself, you are still on the Naegele clock. Do not mix that with an app that started its clock from a positive test date; you will get two gestational ages. Pregnancy apps that show “week 12” from LMP are using the same convention as this page, not a scan. When the app and the clinic disagree, the clinic chart wins.

The calendar is not the scan

Use Due Date Calculator when you want to see LMP + 280 days, optionally shifted for cycle length, without making an account. Then treat the printed day as a teaching EDD. The dating that matters for care is the one on the obstetric record, often from an early ultrasound. The health calculators hub is educational. Prenatal care is a clinic.