How the OB wheel is calculated
A pregnancy (OB) wheel is a dating aid: give it one known date and it reads off the estimated due date and the current gestational age. Below is exactly how each mode in the calculator works. All of it is ordinary calendar arithmetic — no data leaves your device.
1. Last menstrual period (Naegele’s rule)
The classic method counts 280 days (40 weeks) from the first day of the last menstrual period (LMP):
EDD = LMP + 280 days
If the cycle isn’t 28 days, we adjust: EDD = LMP + 280 + (cycle − 28). A 32-day cycle pushes the due date four days later, because ovulation is later. This is the same logic the American College of Obstetricians and Gynecologists (ACOG) describes for menstrual dating.
2. Known due date (reverse dating)
If you already have an EDD from an earlier scan, we work backwards to an equivalent LMP so the rest of the numbers line up: LMP = EDD − 280 days, then gestational age is measured from there.
3. Ultrasound dating
An early scan measures the embryo and reports a gestational age on the scan date. We anchor the pregnancy to that measurement:
dating LMP = scan date − (weeks×7 + days), then EDD = dating LMP + 280 days.
First-trimester ultrasound is generally the most accurate way to date a pregnancy, which is why guidelines favour it when an early scan is available.
4. Conception / ovulation date
Counting from a known conception (or ovulation) date uses 266 days: EDD = conception + 266 days. For the gestational-age display we treat the equivalent LMP as 14 days earlier.
5. IVF
IVF dates are precise because the timing is known. From the transfer date:
- Day-5 (blastocyst) transfer:
EDD = transfer + 261 days - Day-3 (cleavage) transfer:
EDD = transfer + 263 days - Egg retrieval (day 0):
EDD = retrieval + 266 days
These all agree with the general rule equivalent LMP = transfer − (14 + embryo age), then EDD = equivalent LMP + 280.
6. Reverse gestational-age look-up
“When will the patient be 36 weeks?” is just target date = LMP + (weeks×7 + days). The calculator shows the calendar date for any target you type.
Milestones and term categories
From the LMP the wheel marks the start of the second trimester (14w), fetal viability (24w), and ACOG’s term categories: early term 37 0/7–38 6/7, full term 39 0/7–40 6/7, late term 41 0/7–41 6/7, and post-term at or beyond 42 0/7. Trimester boundaries vary between sources; we use 1st = 0–13 weeks, 2nd = 14–27 weeks, 3rd = 28 weeks to birth.
A note on accuracy
Different methods can disagree by a few days, and that’s expected. Only a minority of babies arrive exactly on the estimated date; the EDD is a reference point, not a deadline. See how accurate is a due date for more.
Sources
Frequently asked questions
Which dating method is most accurate?
For most pregnancies, a first-trimester ultrasound (using crown-rump length) is the most accurate way to date a pregnancy. When a reliable early scan is available, professional guidelines generally recommend dating by the scan rather than by the last menstrual period.
Why does the wheel use 280 days?
Naegele’s rule assumes a 40-week (280-day) pregnancy counted from the first day of the last menstrual period, which corresponds to roughly 266 days from conception on an idealised 28-day cycle.
Does cycle length really change the due date?
Yes, for LMP-based dating. A cycle longer than 28 days means ovulation happens later, so the due date shifts later by the same number of days; a shorter cycle shifts it earlier. Ultrasound and IVF dating do not use cycle length.