What This Pregnancy Calculator Does
Enter either the first day of your last menstrual period (LMP) or an already-confirmed due date, and this tool computes where you are right now: gestational age in weeks and days, current trimester, percentage progress, estimated due date, conception window, and days remaining.
Pregnancy dating counts from the LMP — not conception — so the "40 weeks" of pregnancy includes roughly two weeks before conception even occurred. It's a convention that surprises almost everyone, and it's why your doctor's numbers may seem two weeks "ahead" of the calendar in your head.
The Dating Math
Gestational age today = (Today − LMP), shown as weeks + days
Cycle adjustment: for cycles ≠ 28 days, shift by (cycle length − 28)
Example
LMP = March 10, 28-day cycle. Due date = December 15 (March 10 + 280 days). On July 15, gestational age = 18 weeks 1 day — trimester two, about 45% complete, conception estimated around March 24 (LMP + ~14 days).
Ultrasound dating from the first trimester (crown-rump length) is more precise than any calendar math and, when it differs meaningfully, clinicians re-date the pregnancy from the scan.
Trimesters and Key Milestones
| Stage | Weeks | Highlights |
|---|---|---|
| First trimester | 0–13 | Organ formation; heartbeat detectable ~6–7 wks; NT scan 11–13 wks |
| Second trimester | 14–27 | Anatomy scan 18–22 wks; movements felt 16–25 wks; viability threshold ~24 wks |
| Third trimester | 28–40+ | Glucose screen 24–28 wks; GBS test 36–37 wks; full term 39 wks |
"Term" has precise definitions: early term 37–38 weeks, full term 39–40, late term 41, post-term 42+. Only about 4–5% of babies arrive on their exact due date — the due date is the middle of a distribution, not an appointment.
How Accurate Are Due Dates?
- ~4–5% of births occur on the due date itself
- ~70% occur within ±10 days of it
- First babies tend to arrive slightly later on average
- First-trimester ultrasound narrows dating to ±5–7 days; LMP math assumes regular cycles and remembered dates
Treat the due date as a two-week window centered on the estimate. Planning (leave dates, travel cutoffs, nursery deadlines) should assume the window, not the day.
Using Each Mode
Calculate by LMP
The standard first-visit method. Enter LMP and cycle length; if your cycles run long or short, the adjustment shifts conception and due date accordingly.
Calculate by due date
Already have a scan-confirmed due date? Enter it and the tool works backward — current gestational age, trimester, and the week-by-week timeline still apply, now anchored to the better estimate.
Related tools: the Due Date Calculator for quick Naegele-rule dates, and the Conception Calculator for estimating the fertile window behind a known date.
The Bottom Line
One date — your LMP or your due date — unlocks the whole timeline: where you are, what's next, and how far there is to go. Check it weekly if you like watching the progress bar move, but let your prenatal care team own the clinical decisions; calendar math informs, ultrasounds and clinicians decide.
This tool is for information only and doesn't replace prenatal care — always confirm dates and decisions with your healthcare provider.