Due Date Calculation Methods: LMP vs. Ultrasound — Which Is More Accurate?
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Naegele’s Rule: The LMP Method
Naegele’s rule: add 7 days to the first day of your last menstrual period, then add 9 months (or subtract 3 months). This assumes a 28-day cycle with ovulation on day 14. For cycles longer than 28 days, add the extra days. For 35-day cycles, add 7 days to the estimated due date.
Ultrasound Dating: Crown-Rump Length
An ultrasound at 8–12 weeks measures crown-rump length (CRL) — the most accurate dating method. At this stage, gestational age can be estimated to within ±5 days. After 12 weeks, accuracy decreases because fetal size variation increases. First-trimester ultrasound changes the due date for about 15–20% of pregnancies.
Which Method to Trust
First-trimester ultrasound is more accurate than LMP dating when cycle length is irregular, when LMP is uncertain, or when there’s a discrepancy of more than 5 days between LMP and ultrasound dating. Pregnancy Calculator uses Naegele’s rule as the standard with cycle length adjustment.
What ‘Due Date’ Actually Means
Only about 5% of babies are born on their exact due date. The ‘normal’ range is 38–42 weeks. The due date is week 40 — medical management typically begins if labor hasn’t started by 41–42 weeks. Think of it as a due window, not a due date.
The Real Reason People Search For Due Date Calculation Methods: LMP vs. Ultrasound
Most people search for due date calculation methods: lmp vs. ultrasound — which is more accurate? because a small task is blocking a bigger outcome: sending a file, checking a number, cleaning up content, preparing a school or office deliverable, or fixing something quickly on mobile. The useful answer is not theory alone. The useful answer is a clear path from the problem to a working result. After reading the main idea, use Free Bmi Kit with your own input so the article becomes a finished task, not just saved advice.
A 60-Second Workflow You Can Try Now
Start with one realistic example instead of an abstract sample. Confirm the input labels, enter the values or upload the file, review the preview or result, then use copy, export, download, reset, or share only after the output makes sense. This fast workflow is what turns search traffic into real product usage: the reader arrives with a task, sees the exact next step, and can complete it immediately in the browser.
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Mistakes That Make Good Tools Look Wrong
Before trusting the output, check whether the tool expects plain text, numbers, dates, units, files, or a specific format. Recalculate once after changing the main input, compare the result with a simple estimate, and read the labels around the output. Many bad results come from pasted values in the wrong field, hidden units, stale browser state, or rounding too early. The tool should make the work easier, but the final check still belongs to the user.
The Best Next Step
If this article matched your problem, do not leave the idea in the article. Open Free Bmi Kit, try the workflow with one real example, and keep the result only after it passes your own quick check. That is the standard every YantraKosha blog should follow: a useful hook, a real use case, a clear workflow, and a relevant next action.
Quick Reference For Repeat Use
Bookmark Free Bmi Kit so the next time the same task comes up you do not have to search again. Save the input format that worked for you, keep one tested example nearby, and treat the tool as a small reliable step inside your larger workflow. Public tools work best when they fit into a habit, not when they are rediscovered every week from a fresh search result.
Frequently Asked Questions
Try the workflow
Calculate your due date
Open Pregnancy Calculator to estimate your due date, trimester, and week-by-week timeline.