How far along am I, counted from the last period?
Pregnancy is counted from the first day of the last menstrual period, not from conception. That convention surprises almost everyone the first time: on the day conception actually happens, you are already counted as about two weeks pregnant, and a positive test at a missed period usually means about four weeks. The reason is practical. The first day of the last period is a date most people can identify, while the moment of conception is almost never known exactly, so medicine uses the period date as a shared starting line and measures everything from there.
This calculator takes that starting line, counts forward to the date you choose (today, by the example on this page), and reports exactly how far along that is: whole weeks plus the leftover days, the trimester you are in, the estimated due date, and how many days remain. Change the "calculate as of" date to any day, past or future, to answer questions like "how far along will I be at Christmas?" or "how far along was I at the scan?". The companion Due Date Calculator starts from a conception date instead; this page is the one built around where you are right now. Everything here is an estimate only, not medical advice.
The formula: Naegele's rule, adjusted for your cycle
Naegele's rule dates a pregnancy at 280 days, forty weeks, from the first day of the last period, traditionally written as add a year, subtract three months, add seven days. It quietly assumes a 28-day cycle with ovulation on day 14. If your cycle is longer, conception probably happened later, so this page shifts the due date later by the difference: a 30-day cycle adds two days, a 26-day cycle subtracts two. The same adjustment is applied to every milestone date below, so the whole table stays consistent with the due date.
Trimesters and the milestones ahead
The forty weeks are grouped into three trimesters. The first runs to 13 weeks and 6 days (day 97 of the count), the second from week 14 to 27 weeks and 6 days (day 195), and the third from week 28 until birth. On this page the trimester is decided by total days: under 98 days is the first, under 196 the second, and from 196 days onward the third. The milestones table marks the day you reach week 12, the week 20 halfway point, the start of the third trimester at week 28, week 37, which is classed as early term, and week 40 itself. Treat all of them as planning landmarks: scans, tests and appointments are scheduled around weeks like these, but babies follow their own timetable, and only about 1 in 20 arrive on the due date itself.
Worked example
Last period starts September 15, 2026, cycle length 28 days, calculated as of October 12, 2026. Days between the two dates: 27. That is 27 ÷ 7 = 3 weeks and 6 days along, in the first trimester. Due date = Sep 15 + 280 days = June 22, 2027, and days to go = 280 − 27 = 253 days.
How to use this calculator
- Enter the first day of your last period as accurately as you know it.
- Adjust the cycle length if your cycles are usually longer or shorter than 28 days.
- Set the date to calculate as of: today, or any date you want to check.
- Read how far along you are in the headline panel, with your trimester and due date in the table.
- Look at the milestones table for the dates of week 12, week 20, week 28, week 37 and your due date.
Why an ultrasound can move your due date
Calendar dating assumes your cycle matched the textbook: 28 days, ovulation on day 14, dates remembered exactly. Real life often differs. Cycles vary, ovulation drifts, and period dates are sometimes misremembered by a few days, each of which shifts the true due date. An early ultrasound measures the developing baby directly, and in the first trimester that measurement dates a pregnancy more accurately than the calendar can, so clinics commonly set or revise the official due date after the first scan. If that happens, it is not that anyone made a mistake: better information has replaced an estimate, and the revised date is the one your care team will plan around.
Hold every date on this page the same way. A due date is the middle of a several-week window in which birth is normal, not an appointment and not a deadline your body has agreed to. Use the week count to follow along with development information and to schedule the practical things, and bring any real question or worry, such as bleeding, pain, reduced movements, or simply not knowing your dates, to a qualified health professional promptly rather than to a calculator. If your dates are uncertain, the scan, not this page, is the authority. Estimates only, not medical advice.
Important note
For information only — not medical advice. This calculator uses standard published formulas to estimate values for general education and planning. It does not diagnose, treat, or replace the advice of a doctor or other qualified health professional. If you have a health concern, are pregnant, or are making decisions about treatment, medication, or a major diet or exercise change, please talk to a healthcare professional first.