Pregnancy Calculator
Estimate your due date, current gestational week, and trimester from the first day of your last menstrual period, average cycle length, and a reference date.
Calculator Inputs
The 4-digit year of the first day of your last menstrual period.
The month of the first day of your last menstrual period (1-12).
The day of the first day of your last menstrual period (1-31).
Your typical cycle length in days (21-35).
The 4-digit year to calculate gestational age as of.
The month to calculate gestational age as of (1-12).
The day to calculate gestational age as of (1-31).
- Estimated Due Date — Day
- 8
- Gestational Age — Completed Weeks
- 0
- Estimated Due Date — Year
- 2026
- Estimated Due Date — Month
- 10
- Gestational Age — Additional Days
- 0
- Trimester
- 1
About these results
CalcSpectrum calculators produce automated estimates using established formulas and the specific values you enter. Because real-world outcomes depend on input accuracy, mathematical assumptions, and rounding conventions, please verify critical figures with an appropriately qualified financial, tax, medical, legal, or other professional, as relevant.
Health & Fitness Notice: Wellness and body metric calculations provide informational estimates based on generalized population formulas. They do not provide medical diagnosis, clinical advice, or treatment plans. Consult an appropriately qualified medical or healthcare professional for personal guidance.
How It's Calculated
Formula
This calculator uses the same modified Naegele's rule as the Due Date Calculator to estimate your due date from the first day of your last menstrual period (LMP) and your average cycle length. It also computes your gestational age — completed weeks and days — as of a reference date you provide, and reports which trimester that falls in (first trimester: weeks 0-13, second trimester: weeks 14-27, third trimester: week 28 onward, the standard three-trimester convention used in patient-education materials). The reference date is an explicit input rather than the current date, so results stay deterministic and reproducible from a shared link. This is an ESTIMATE based on a published formula, not a guaranteed birth date or a confirmation of current pregnancy status — most births occur within a range of days around the estimated due date.
Worked Examples
28-day cycle, LMP January 1, 2026, reference date March 15, 2026
- Days elapsed: Jan 1 to Mar 15, 2026 = 73 days
- Completed weeks: 73 ÷ 7 = 10 weeks, remainder 3 days
- Gestational age: 10 weeks, 3 days — first trimester (≤13 weeks)
- Due date (Naegele's rule): Jan 1, 2026 + 280 days = Oct 8, 2026
28-day cycle, LMP January 1, 2026, reference date July 16, 2026
- Days elapsed: Jan 1 to Jul 16, 2026 = 196 days
- Completed weeks: 196 ÷ 7 = 28 weeks, remainder 0 days
- Gestational age: 28 weeks — third trimester (≥28 weeks)
Frequently Asked Questions
Why does this calculator ask for a reference date instead of using today's date?
Using an explicit reference date instead of the browser's current date keeps the calculation deterministic and reproducible — the same inputs always produce the same result, whether calculated today or from a saved/shared link.
How is gestational age different from fetal age?
Gestational age is counted from the first day of your last menstrual period, which is roughly two weeks before conception typically occurs. This calculator reports gestational age, the standard obstetric convention, not fetal age.
What are the trimester week ranges?
First trimester: weeks 0 through 13. Second trimester: weeks 14 through 27. Third trimester: week 28 onward. These are the standard calendar boundaries used in patient-education materials, counted in completed weeks of gestational age.
Does this calculator confirm I am pregnant or how far along I actually am?
No. This is a calendar-based estimate from the dates you enter, not a medical confirmation of pregnancy or gestational age. Consult a healthcare provider for individualized assessment, such as an ultrasound-based estimate.