Accurate Conception Calculator
Estimate your conception date and fertility window from last period, due date, or ultrasound. Adjust for your cycle length for better precision.
Ready to Get Started?
Hey, pick your starting point – last period date works for most, or plug in your due date if you got that from a doctor visit. Cycle length tweaks it for you, since not everyone’s on 28 days. Hit calculate, and you’ll see the likely conception day plus a window when things probably happened.
- Got irregular periods? Bump up the cycle days for a wider window.
- Know your ovulation? That lines up right in the middle of the range shown.
- Fertility tracking apps? Their peak days match this output spot-on.
What’s Happening Behind the Math?
Picture this: ovulation kicks in about 14 days into a standard cycle, give or take. Sperm hangs around 3-5 days, so the window spans back from there. From LMP, add cycle minus 14 for conception day. Due date? Subtract 266 days (38 weeks). Ultrasound? Straight gestational age minus 2 weeks to conception. All backed by standard medical math.
Real talk – exact day is rare, but this narrows it to a 5-day span most times.
Got Questions? Let’s Chat
Sperm survives 3-5 days, egg 24 hours – conception can happen anytime in that overlap. Your range covers it all.
Select your average length; the calc shifts ovulation accordingly. Still off? Ultrasound trumps all.
Yep, first trimester scans nail it best. Enter as weeks + days pregnant.
Not directly, but if delivery was full-term, subtract 40 weeks from birth for LMP estimate.
LMP vs Due Date vs Scan – Which Wins?
| Method | Best For | Accuracy | Window Size |
|---|---|---|---|
| LMP | Regular cycles | ±5 days | 5-7 days |
| Due Date | Known EDD | ±7 days | 5 days |
| Ultrasound | Early pregnancy | ±3 days | 3-5 days |
Early ultrasound edges out for precision, especially if cycles vary. LMP’s quick but assumes day 14 ovulation.
Common Mix-Ups Cleared Up
Think pregnancy starts at conception? Nope, docs count from LMP, so gestational age is 2 weeks ahead. Myth: all cycles ovulate day 14 – truth, it shifts with length. Another: sperm dies quick – actually sticks 5 days max. Busts those “must be exact” worries.