Why Your Due Date Keeps Changing (And Which Estimate to Trust)
If your last-period date, your first ultrasound, and your IVF transfer date all point to slightly different due dates, you're not doing anything wrong — each method estimates a different thing. Here's how to make sense of the numbers your pregnancy due date calculator gives you.
Why do different dating methods give different due dates?
Short answer: Each method estimates conception timing from a different signal — a remembered period date, a measured embryo size, or a known transfer date — so small variations in ovulation timing or cycle length show up as a few days' difference between methods.
The default method, Naegele's rule, adds 280 days to the first day of your last menstrual period (LMP). It assumes a textbook 28-day cycle with ovulation on day 14. In practice, cycles vary — some people ovulate on day 10, others on day 20 — so the LMP method is really an average-case estimate, not a measurement of anything that actually happened in your body that month.
An ultrasound, by contrast, measures the embryo or fetus directly (usually crown-rump length in the first trimester) and infers gestational age from that measurement. Because it's based on observed growth rather than a remembered date, it tends to be treated as the more reliable estimate — particularly for anyone with irregular cycles or an uncertain LMP.
LMP-based vs. ultrasound-based due dates: an example
Short answer: If your first-trimester ultrasound estimate and your LMP-based estimate disagree by more than about a week, many providers will use the ultrasound date going forward — it's considered the more direct measurement.
Here's an illustrative (not a real patient's) example of how the two methods can diverge:
Example only — illustrative numbers, not a real case
Dating method
Input
Estimated due date
LMP (Naegele's rule)
Last period: March 1
~December 5
Ultrasound (8w0d at scan)
Scan measured 8 weeks 0 days
~December 9
Conception date
Conception ~March 15
~December 6
A gap of a few days like this is common and not a cause for concern on its own — it simply reflects the natural variation between a calendar-based estimate and a growth-based measurement. Your due date calculator lets you compare all of these side by side using the LMP, conception, ultrasound, and IVF tabs.
Worth remembering: only an estimated 5% of babies are born exactly on their calculated due date. A due date is best thought of as the midpoint of a normal range, not a deadline.
How the IVF due date formula actually works
Short answer: For a day-5 (blastocyst) embryo transfer, the due date is the transfer date plus 261 days. For a day-3 (cleavage-stage) transfer, it's transfer date plus 263 days. The two-day gap simply accounts for the embryo being two days younger at transfer.
Because IVF timelines are tracked precisely — the exact day the embryo was created and the exact day it was transferred are both known — the IVF-based due date removes most of the guesswork involved in the LMP method. This is one reason IVF pregnancies are often dated with more initial confidence than naturally conceived ones.
Gestational vs. fetal age: Gestational age counts from the last period (or its IVF/ultrasound equivalent); fetal age counts from actual conception, roughly two weeks later. At "12 weeks pregnant" by gestational age, the embryo's developmental age is closer to 10 weeks.
Reading your current week, trimester, and countdown
Short answer: Once you calculate your due date, the tool also shows your current gestational week and day, which trimester you're in, and a countdown of days remaining — plus a week-by-week view of your baby's estimated size from week 4 through week 40.
Gestational age is simply the number of days since your reference date, divided by seven. The three trimesters break down roughly as weeks 1–13, 14–27, and 28–40, and each one lines up with a different set of routine prenatal screenings (NT scan, quad screen, anatomy ultrasound, glucose tolerance test, and so on). If you want those screening windows converted into actual calendar dates for your own due date, the calculator's personalized calendar section does that automatically and lets you export it as an .ics file for your phone's calendar app.
Practical scenario (example only): say your LMP-based due date works out to December 5. Entering that date into the tool would show you're currently in a specific week and trimester, how many days remain, and roughly when the anatomy ultrasound and glucose test windows fall — all as real dates you could add to a calendar, rather than abstract week numbers.
Compare LMP, conception, ultrasound, and IVF due dates side by side, and see your week-by-week tracker.
Ultrasound dating in the first trimester is generally considered more precise than counting from your last period, especially if your cycle is irregular. If the scan measurement disagrees with your LMP-based date by more than about 5-7 days in the first trimester, many providers will re-set the due date to the ultrasound estimate.
Is the IVF due date more accurate than an LMP-based one?
Yes, generally. Because the embryo transfer date and embryo age (day 3 or day 5) are known precisely, an IVF-based due date removes the guesswork of estimating ovulation timing that LMP-based dating relies on.
How many weeks pregnant am I if I don't know my exact LMP date?
If your last period date is uncertain, an ultrasound measurement from earlier in pregnancy is usually the most reliable way to estimate your gestational age, since it's based on measured fetal size rather than a remembered date.
Why does the calculator say I'm "pregnant" during weeks 1 and 2?
Gestational age is counted from the first day of your last period, not from conception. Conception typically happens around two weeks after that date, so the first two weeks of the pregnancy calendar occur before fertilization actually happens.
Does this calculator save or send my dates anywhere?
No. All calculations run locally in your browser and nothing is sent to a server. The only way a result becomes shareable is if you choose to copy a generated link, and sharing that link is entirely your decision.
A note on methodology: the figures and formulas in this article (Naegele's rule, IVF day-3/day-5 offsets, trimester week ranges, prenatal test windows) reflect commonly used obstetric conventions, and any example numbers above are clearly labeled as illustrative, not real data. This article is for general informational purposes only and is not a substitute for guidance from your doctor or midwife.