What the pregnancy calculator does
This tool answers the two questions almost every expecting parent asks first: when is the baby due, and how far along am I today? From a single piece of information, the first day of your last menstrual period (LMP), it estimates your due date, how many weeks and days pregnant you are right now, which trimester you’re in, and how many days are left before you meet your baby.
Everything is calculated inside your browser. No date is stored or uploaded anywhere, so you can use it with complete privacy.
How to use the calculator
- Choose a calculation method. The standard approach starts from your last menstrual period; if you know the exact conception date (for example, after a fertility treatment), switch to that mode.
- Pick the date on the calendar. Future dates are not allowed.
- Read the estimated due date on the main card.
- Check your weeks and days of pregnancy, your current trimester, and the days left on the cards below.
If the result shifts when you change methods, remember that both point to the same day: conception happens about two weeks after the last period, and the tool already adjusts for that gap.
How it’s calculated: the Naegele rule
The standard method in obstetrics is the Naegele rule: add 280 days (40 weeks) to the first day of your last menstrual period. That result is your estimated due date.
Here’s the part that surprises most people: pregnancy is counted from the last period, not from conception. That’s why it’s described as “40 weeks” even though the baby has biologically only been developing for about 38 of them. The first two weeks of the count pass before fertilization ever happens. If you prefer to start from conception, the count is 266 days (280 minus 14).
Worked example
Say your last menstrual period started on March 3, 2026. Add 280 days:
- March 3 + 280 days = December 8, 2026. That’s your estimated due date.
If today were June 15, 2026, then 104 days would have passed since the LMP, which is 14 weeks and 6 days of gestation. You’d be well into your second trimester, with roughly 176 days left until the estimated date.
Weeks by trimester
| Trimester | Gestational weeks | What’s happening, broadly |
|---|---|---|
| First | 1 to 13 | Major organs form; morning sickness is common. |
| Second | 14 to 27 | Symptoms often ease; first movements are felt. |
| Third | 28 to 40 | The baby gains weight fast; the body prepares for birth. |
How accurate is the date
An estimated due date is exactly that: an estimate. Only about 4% of babies are born on the precise calculated day. The normal window is wide: a pregnancy is considered full term between weeks 37 and 42. Arriving a few days before or after your due date is completely expected.
The most reliable dating comes not from this calculator but from a first-trimester ultrasound, which measures the baby and refines the date, especially when your cycles are irregular.
Frequently asked questions
Why is it counted from the last period instead of conception?
Because almost no one knows the exact day they conceived, but most people remember when their last period started. Medicine adopted that point because it’s the easiest to identify, which is why a “40-week” pregnancy includes the two weeks before fertilization.
Is the due date exact?
It’s a statistical estimate, not a guarantee. Only a small share of babies are born on the calculated day. Treat it as a planning reference, not a fixed appointment on the calendar.
My cycles are irregular. Does it still work?
It works as a starting point, but it will be less precise. The Naegele rule assumes a regular 28-day cycle with ovulation on day 14. If your cycles vary, the first-trimester ultrasound should override the date.
When is a pregnancy considered full term?
Between 37 and 42 completed weeks. Before week 37 it’s called preterm, and after week 42 it’s post-term; in either case your doctor will decide on the right follow-up.
Does this replace prenatal care?
No. It’s an informative tool to accompany your pregnancy, never a substitute for checkups, tests, or your doctor’s judgment. Any question or symptom should be evaluated by a healthcare professional.