Find out how many weeks and days pregnant you are, and your current trimester.
Fill in the fields on the left, then press Calculate to see your result instantly. This tool provides general estimates for informational purposes and does not replace advice from your doctor or midwife.
Pregnancy is traditionally measured from the first day of your last menstrual period (LMP), not from the actual date of conception, which usually occurs about two weeks later around ovulation — a dating convention established because LMP is typically a known, memorable date, while the exact date of conception generally isn't. This calculator counts the days since your LMP to tell you exactly how many weeks and days pregnant you are, which trimester you're in (first trimester: weeks 1–13, second: weeks 14–27, third: weeks 28–40), and your estimated due date based on a standard 280-day (40-week) pregnancy length using Naegele's rule. Because this dating convention starts counting two weeks before conception even happened, a pregnancy that's "4 weeks along" by LMP dating is really only about 2 weeks past actual conception — a distinction that sometimes confuses early pregnancy timelines but is universally used by healthcare providers for consistency, since it doesn't depend on knowing an exact, often-unknowable conception date.
New expectant parents are often surprised to learn that the "weeks pregnant" number their doctor uses doesn't start counting from conception at all — it starts from the first day of the last menstrual period (LMP), typically about two weeks before conception actually occurs. This can make early pregnancy math feel oddly offset until the underlying convention is understood.
The reason LMP was adopted as the universal starting point, rather than conception date, comes down to practicality: most people know the date their last period started with reasonable confidence, while the exact date of conception is rarely known with certainty even by people actively trying to conceive, since ovulation timing varies and pinpointing the precise fertilization moment isn't possible without invasive tracking.
This means that by the time a pregnancy is "officially" 4 weeks along by standard dating, actual fetal development has only been underway for about 2 weeks since fertilization — an important distinction for understanding early pregnancy milestones and why very early developmental descriptions sometimes seem to lag behind the stated week count.
Trimesters are similarly defined by convention using this LMP-based counting system: the first trimester spans weeks 1 through 13, the second spans weeks 14 through 27, and the third spans weeks 28 through 40 (or until delivery). These aren't precise biological boundaries so much as widely agreed-upon reference points that every healthcare provider uses consistently, which is exactly the value of having a single standardized system.
For anyone with irregular cycles, where actual ovulation timing may deviate meaningfully from the standard 14-day-after-LMP assumption, LMP-based dating alone can be somewhat less precise, which is part of why an early first-trimester ultrasound is often used to confirm or adjust the estimated dating — measuring the fetus directly provides a check on the calendar-based calculation, particularly useful for anyone whose cycle doesn't closely match the standard 28-day assumption this calculator's default estimate is built around.
Because the exact conception date is rarely known, while the start of your last period is a clear, memorable reference point used universally in obstetrics.
This calculator uses the standard 280-day estimate; your doctor may adjust your dates using an early ultrasound if your cycle is irregular.
Because LMP-based dating starts the clock about two weeks before conception typically occurs, a pregnancy described as "4 weeks" is really only about 2 weeks past fertilization. This gap is a normal feature of standard medical dating convention, not an error or inconsistency in how your specific pregnancy is being counted.