Pregnancy Week Calculator — How Many Weeks Pregnant Am I?

Why Due Dates Are Always Estimates

A due date is a population-level reference point, not a scheduled arrival. Only about 5% of birthing people deliver spontaneously on their calculated due date, and roughly 80% deliver within a ±2-week window around it. Variation comes from normal biological differences in implantation timing, menstrual-cycle length, fetal growth patterns, and individual factors we do not fully measure with simple calculators. Your prenatal care team will adjust your official dating based on first-trimester ultrasound crown-rump measurement when needed, which is the most accurate reference window we have.

Method 1 — Naegele Rule (LMP + 280 Days)

Method 2 — Conception Date

If you know the approximate date of ovulation, intercourse, or IUI or IVF fertilization, a conception-date estimate adds 266 days (38 weeks, the average duration of human gestation from fertilization to term) instead of 280. This removes the two-week LMP-to-conception offset, so the raw calculation does not depend on cycle length. Keep in mind that sperm can survive inside the female reproductive tract for several days, so if you are estimating from intercourse the actual fertilization window can span multiple days.

Method 3 — IVF Transfer

Cycle Length and LMP Dating

Naegele rule assumes a 28-day cycle. For people with consistently longer or shorter cycles, a simple linear adjustment (shifting the estimate by [cycle length − 28] days) can be applied. However, because luteal-phase length is relatively stable across cycle lengths (about 14 days) while follicular phase varies, adjusting LMP-based estimates purely for cycle length is an approximation at best. For irregular cycles, conception-date or first-trimester-ultrasound dating is generally more reliable.

Trimester Split

Three standard trimesters are used for reference and prenatal-care scheduling. The first trimester runs from LMP through week 12 + 6 days and covers the period of highest sensitivity for embryological development. The second trimester spans week 13 through week 27 + 6 days and is the typical window for anatomical screening, prenatal sugar screening, and fetal-growth surveys. The third trimester runs from week 28 through delivery, with the viability reference point commonly placed at 24 weeks and the term reference window at 37–42 weeks.

How Pregnancy Weeks Are Counted

Pregnancy is counted in gestational weeks, which start from the first day of the last menstrual period (LMP), not from conception. This convention means that by the time fertilization happens (around day 14 of a 28-day cycle), you are already counted as 2 weeks pregnant. A full-term pregnancy is 40 gestational weeks (280 days from LMP, or 266 days from fertilization). The current gestational age equals the number of completed weeks and days between LMP and today, expressed as "X weeks and Y days." For example, 21 weeks and 4 days means 21 full weeks have elapsed plus 4 additional days into week 22. This counting convention is universal in prenatal records, so the week number your care team uses matches the number this calculator produces.

Trimester Breakdown

The 40-week pregnancy is divided into three trimesters, each with its own developmental milestones and prenatal visit cadence:

The viability reference point is commonly placed at 24 weeks, when survival outside the womb becomes possible with intensive support. The term reference window runs from 37 to 42 completed weeks — births in this window are considered "term." Births before 37 weeks are "preterm," and births after 42 weeks are "post-term."

Weekly Development Milestones

Each gestational week marks specific developmental milestones. A few widely used reference points: the heartbeat is typically detectable by transvaginal ultrasound around 6 to 7 weeks; the neural tube closes by week 6 to 7; the crown-rump length is the most accurate dating reference at 8 to 12 weeks; the anatomy scan is scheduled at 18 to 22 weeks; quickening (the first movement felt by the pregnant individual) usually happens between 16 and 24 weeks; lung maturation continues through the third trimester with major gains after 34 weeks; and the term reference window begins at 37 weeks. Your prenatal care team will schedule visits and screening checks against these week-based milestones, so knowing your current gestational age orients you to what to expect at each visit.

Gestational Age vs Fetal Age

Gestational age (counted from LMP) and fetal age (counted from fertilization) differ by about two weeks because the standard counting convention starts the clock before conception. When your care team says "you are 12 weeks pregnant," they mean 12 gestational weeks from LMP — the fetus has been developing for about 10 weeks. This calculator reports gestational age, which matches the convention used in prenatal records, ultrasound reports, and published pregnancy timelines. If you are reading about fetal development, check whether the source uses gestational age or fetal age — most consumer-facing pregnancy resources use gestational age, but some embryology sources use fetal age.

Pregnancy Weeks and the Due Date

The due date is the date at which exactly 40 gestational weeks (280 days from LMP) have elapsed. Once the due date is set — by LMP, conception date, IVF transfer, or first-trimester ultrasound — the current gestational age is just the difference between today and the LMP equivalent. Only about 5 percent of pregnancies deliver spontaneously on the calculated due date; roughly 80 percent deliver within a ±2 week window around it. View the due date as a reference point rather than a scheduled arrival, and use the week number to track milestones and visit timing.

Practical Reference Tips

• Gestational age is counted from LMP, not conception — so at fertilization you are already 2 weeks pregnant by convention.
• Express your current gestational age as "weeks + days" (e.g., 21 weeks + 4 days) to match the format used in prenatal records and ultrasound reports.
• Confirm the dating at your first prenatal ultrasound. A first-trimester crown-rump length measurement is the most accurate reference, with a margin of about 5 to 7 days.
• Use the trimester breakdown to orient yourself on milestones and visit cadence — first trimester ends at 13+6, second at 27+6, third runs to delivery.
• View the due date as a reference window (37–42 weeks is term) rather than a single scheduled arrival day.