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Calculate pregnancy weeks and dates, or compare LMP and ultrasound dating against ACOG discrepancy thresholds.
Last updated: July 22, 2026
Build a pregnancy timeline from one established dating source, or compare LMP and ultrasound dates using ACOG discrepancy thresholds.
Guide
Clinical gestational age is conventionally counted from the first day of the last menstrual period, not from fertilization. That convention places the estimated start about two weeks before ovulation in a typical 28-day cycle. The calculator therefore reports completed weeks and remaining days from LMP, while making clear that conception age and gestational age are different measurements.
When LMP is selected, the calculator counts elapsed calendar days from the entered LMP to the reference date. It divides that total by seven for pregnancy weeks and adds 280 days to estimate a due date. The 280-day convention represents 40 weeks from LMP and assumes a cycle close to 28 days; irregular cycles can make that estimate less reliable.
When an estimated due date is already documented, gestational age on a reference date is calculated as 280 days minus the number of days remaining until the EDD. This method is useful for reconstructing pregnancy age on an appointment or test date, but the calculator does not decide whether the documented EDD should be changed.
First-trimester ultrasound is the most accurate ultrasound period for establishing or confirming gestational age. Comparison mode selects ACOG's discrepancy threshold from the LMP-based gestational age on the scan date—not from the ultrasound estimate being compared. It flags clinical review rather than automatically changing an established EDD.
The CRL option applies the published Robinson–Fleming first-trimester equation to a crown-rump length in millimeters. IUI and known conception dates use the conventional two-week gestational offset, while day-3 and day-5 transfer modes use embryo age plus fourteen days. A fertility clinic or ultrasound report remains the authoritative dating record.
Calculation steps
Step 1
LMP 2026-04-01; reference 2026-07-15
Step 2
Clinical gestational age is conventionally counted from the first day of the last menstrual period, not from fertilization. That convention places the estimated start about two weeks before ovulation in a typical 28-day cycle. The calculator therefore reports completed weeks and remaining days from LMP, while making clear that conception age and gestational age are different measurements.
Step 3
LMP convention: Gestational day 0 is the first day of LMP, adjusted for the entered cycle length. Conception, IUI, and IVF: Conception and IUI begin at 2w0d; embryo-transfer methods add embryo age plus 14 days. CRL estimate: Crown–rump length from 2–84 mm is converted with the Robinson–Fleming equation and should be treated as an estimate, not a substitute for the signed ultrasound report. Ultrasound thresholds: The comparison uses ACOG's pregnancy-week-specific discrepancy thresholds, selected from gestational age by LMP at the scan, and flags review only when the difference is greater than the stated number of days. Output range: Dates outside 0–42 weeks are rejected as likely input errors.
Step 4
15 weeks, 0 days; EDD 2027-01-06. The interval is counted from the first day of LMP.
Timeline mode uses one selected dating source. Comparison mode transparently reviews LMP and ultrasound EDD differences without replacing clinical judgment.
| Assumption | How this page handles it |
|---|---|
| LMP convention | Gestational day 0 is the first day of LMP, adjusted for the entered cycle length. |
| Conception, IUI, and IVF | Conception and IUI begin at 2w0d; embryo-transfer methods add embryo age plus 14 days. |
| CRL estimate | Crown–rump length from 2–84 mm is converted with the Robinson–Fleming equation and should be treated as an estimate, not a substitute for the signed ultrasound report. |
| Ultrasound thresholds | The comparison uses ACOG's pregnancy-week-specific discrepancy thresholds, selected from gestational age by LMP at the scan, and flags review only when the difference is greater than the stated number of days. |
| Output range | Dates outside 0–42 weeks are rejected as likely input errors. |
| Input | Output | Why it matters |
|---|---|---|
| LMP 2026-04-01; reference 2026-07-15 | 15 weeks, 0 days; EDD 2027-01-06 | The interval is counted from the first day of LMP. |
| Ultrasound age 10w0d; EDD differs from LMP by 8 days | Clinical dating review indicated | At 9w0d–13w6d, ACOG's stated threshold is more than 7 days. |
| LMP and reference are the same date | 0 weeks, 0 days | This is the conventional start of gestational dating. |
Edge cases
LMP dating assumes timing that may not fit long, short, or irregular cycles.
Use the embryo-transfer and embryo-age method documented by the fertility clinic.
Dating from 28 weeks is least reliable and needs review of growth, prior records, and the complete clinical picture.
Use it to reconstruct an educational pregnancy timeline from an established LMP, due date, conception, IUI, embryo-transfer, scan-age, or CRL input, or to compare LMP and ultrasound dates against a published review threshold.
Do not use it to establish or revise a clinical due date, assess fetal growth or viability, or make prenatal-care decisions without the clinician and signed ultrasound or fertility record.
No. Gestational age is usually about two weeks ahead because it starts from LMP.
No. The result flags clinical review; an established EDD should only be revised by the care team after reviewing all dating evidence.
Yes—select the EDD method and enter the due date documented by the care team.
Use a clinically established EDD, conception, IUI or transfer date, or the known scan age documented by the care team.
Yes. The LMP method adjusts the 280-day convention by the difference from a 28-day cycle.
We use these sources to define the age-counting rule, official context, or life-stage assumptions used by this calculator.
Last updated: July 22, 2026
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