How it works
When you're staring at a positive test and wondering "how far along am I?", this tool answers the question clinicians call gestational age. Rather than predicting a single arrival day, it pinpoints where you stand right now in the 40-week pregnancy timeline — your current week, completed days, and which trimester you've entered.
The method is built on Naegele's rule, established in the 1800s and still the global clinical standard. It takes the first day of your last menstrual period (LMP) as day zero, then counts forward. The full pregnancy spans 280 days, or 40 weeks, from that anchor point.
Here's the key distinction: gestational age counts from your period, not from conception. Since ovulation typically happens around day 14 of a 28-day cycle, you're already considered "two weeks pregnant" at the moment of fertilization. This convention is universal in obstetrics — your midwife, your ultrasound tech, and your medical records all use LMP-based counting.
| Trimester | Week range | What's happening | |---|---|---|---| | First | 1–13 | Organ formation, heartbeat begins | | Second | 14–27 | Anatomy scan, movement felt | | Third | 28–40 | Growth accelerates, position shifts |
This calculator focuses on the "where am I today" question. You enter your LMP, and it measures the span to the current date, then reports your gestational age in weeks and days (clinicians speak this way — "23 weeks 4 days" rather than a decimal).
The formula
Gestational age (days) = Current date − LMP date; weeks = floor(days ÷ 7); remaining days = days mod 7
Worked example
Suppose today is 2025-03-10 and your LMP began on 2024-12-11.
Days elapsed: 2025-03-10 − 2024-12-11 = 89 days
Completed weeks: 89 ÷ 7 = 12 with remainder 5
Gestational age: 12 weeks 5 days
At 12 weeks 5 days, you sit in the first trimester (weeks 1–13). You're days away from crossing into the second trimester, when many people notice early pregnancy discomfort easing.
Due date (Naegele's): 2024-12-11 + 280 days = 2025-09-17
That puts your estimated delivery at mid-September 2025 — roughly six months from today.
Things to watch
Irregular cycles. Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14. If your cycles run 35 days, ovulation shifts later by a week, meaning your gestational age would be overestimated by about seven days. An early dating ultrasound (typically offered between 8 and 14 weeks) measures the fetus directly and corrects for cycle variation far more accurately than calendar math.
Spotting vs. a true period. Implantation bleeding around the time you'd expect a period can cause you to log the wrong LMP. If what you recorded as your period was unusually light or brief, mention it to your provider — it may shift your dates.
The two-week buffer. Don't be alarmed that you're "two weeks pregnant" before any conception could have occurred. This is simply the clinical convention. Everyone's gestational clock starts at the LMP.
This tool produces an estimate, not professional medical advice. Your prenatal care team may adjust your dates based on ultrasound measurements, fundal height, or other clinical factors. Always defer to your healthcare provider for decisions about your pregnancy.