AwaitedHow Due Dates Are Calculated: LMP, Ultrasound, and IVF
Your "due date" is one of the first numbers you learn in pregnancy, and one of the most misunderstood. It is not a delivery appointment; it is a statistical estimate — the midpoint of a range in which the great majority of healthy, full-term babies arrive. This guide explains the three main ways due dates are estimated, how gestational age is counted, and why thinking in terms of a delivery window is both more accurate and kinder to your nerves.
First, how pregnancy is counted
Gestational age is conventionally counted from the first day of your last menstrual period (LMP) — not from conception. That means by the standard count, you are already "4 weeks pregnant" around the time of a missed period, even though fertilization happened roughly two weeks earlier. A full-term pregnancy is counted as about 40 weeks (280 days) from LMP, which corresponds to roughly 38 weeks from conception.
Gestational age is usually written as weeks plus days — "32+4" means 32 weeks and 4 days. Understanding this convention makes every appointment, scan report, and app screen easier to read.
Method 1: LMP and Naegele’s rule
The classic calculation, taught for two centuries, is Naegele’s rule: take the first day of your last period, add one year, subtract three months, and add seven days. That is arithmetic shorthand for "add 280 days to your LMP."
Example: LMP of March 10 → due date around December 17.
LMP dating is simple and requires no equipment, but it has known weaknesses: it assumes you remember the date accurately, that your cycles are about 28 days long, and that ovulation happened around day 14. Longer, shorter, or irregular cycles shift the real fertilization date, which is why LMP estimates are often adjusted later.
Method 2: Ultrasound dating
An early ultrasound estimates gestational age by measuring the embryo or fetus — most commonly the crown-rump length in the first trimester. Because early embryos grow at a remarkably consistent rate, a first-trimester scan is widely considered the most accurate dating method, and professional guidance commonly allows the scan estimate to replace the LMP estimate when the two disagree by more than a defined margin. Dating from later scans is progressively less precise, because healthy babies vary more in size as pregnancy advances.
This is why your "official" due date can change after your first scan — the date was re-estimated with better data, not because anything is wrong.
Method 3: IVF and known conception dates
IVF pregnancies are the special case where the embryo’s age is genuinely known. Dating is calculated from the egg retrieval or embryo transfer date, adjusted for the embryo’s age at transfer (a day-3 transfer and a day-5 blastocyst transfer are counted differently). Because there is no guessing about ovulation, IVF due dates are typically the most precise of all — though delivery still happens across the same natural window.
Why a window beats a date
Here is the number that reframes everything: only a small minority of babies — commonly cited as around one in twenty — arrive on their exact due date. The clinically meaningful range is 37 to 42 weeks:
| Term label | Weeks |
|---|---|
| Early term | 37+0 – 38+6 |
| Full term | 39+0 – 40+6 |
| Late term | 41+0 – 41+6 |
| Post term | 42+0 and beyond |
Treating the due date as the center of a five-week window has practical benefits: you plan leave and logistics realistically, you avoid the "overdue" dread that starts the moment a single date passes, and friends and family get a range instead of a date to pester you about.
How Awaited handles due dates
Awaited’s free due date calculator supports all of the methods above — LMP, ultrasound dating, IVF transfer date, and manual entry if your provider has already given you an official date. Deliberately, the app shows the estimated delivery window (37–42 weeks) alongside the single date to keep expectations realistic and reduce anxiety, and its countdown displays the time remaining in days, weeks, and months with your current trimester and gestational age. All of it stays 100% local on your device — no account, no cloud, no data selling.
Download on theApp StoreQuick answers
Why did my due date change after my first scan?
Because first-trimester ultrasound measures the embryo directly, it often produces a more accurate estimate than LMP arithmetic. Providers commonly adopt the scan date when it differs from the LMP date by more than a set number of days. A revised date is routine, not a red flag.
My cycles are irregular — is my LMP due date useless?
Not useless, but less reliable, since Naegele’s rule assumes ovulation around cycle day 14. Mention irregular cycles to your provider; an early dating scan usually settles it.
Do twins change the due date?
The 40-week estimate is calculated the same way, but multiple pregnancies are often delivered earlier, and care plans differ. Your provider will discuss timing specific to your situation — and if you are tracking twins in an app, look for one that supports multiple profiles.