Pregnancy Calculator — Due Date, Weeks Pregnant & Trimester

The pregnancy calculator estimates your due date, current gestational age in weeks and days, trimester, days remaining until birth, and a complete milestone timeline — from conception through key prenatal screenings to the expected delivery date. It supports three input methods: last menstrual period (LMP, the standard clinical approach), known conception date, or a confirmed due date from a prior ultrasound or provider visit.

Pregnancy is measured in gestational weeks counted from the first day of the last menstrual period, not from conception. This means that by the time a positive pregnancy test appears (typically 4–5 weeks from LMP), you are already about a month into your gestational count. The 40-week pregnancy framework based on Naegele's rule — the formula used by this calculator — has been the clinical standard since the early 1800s, and while ultrasound can refine the dating, the 40-week LMP framework remains the primary reference for scheduling all prenatal care.

Knowing your gestational age at any given moment is practically important: it tells you which trimester you are in, which prenatal screenings are due or upcoming, what fetal development milestones to expect, and how many days remain until your estimated delivery. This calculator provides all of these from a single date input, allowing you to quickly orient yourself in the pregnancy timeline at any stage.

Example 1 — LMP input: LMP: February 1, 2025 Due Date: February 1 + 280 days = November 8, 2025 Conception estimate: February 15, 2025 At week 12 (April 26, 2025): End of first trimester At week 20 (June 21, 2025): Anatomy scan due At week 28 (August 16, 2025): Third trimester begins At week 37 (October 18, 2025): Full-term threshold

Example 2 — Known due date input: Due Date provided: October 15, 2025 LMP equivalent: October 15 − 280 = January 8, 2025 Conception estimate: January 22, 2025 Current gestational age (if today = July 18, 2025): Days from LMP = 191 → 27 weeks 2 days → Third Trimester Days remaining: October 15 − July 18 = 89 days

Knowing your gestational age accurately is the foundation of all prenatal care scheduling. Every screening test in pregnancy has an optimal gestational window — outside that window, the test either cannot be performed or loses its clinical utility. The nuchal translucency scan for chromosomal abnormality detection must be performed between 11 weeks 0 days and 13 weeks 6 days — a 21-day window. The anatomy scan for fetal structural assessment is most informative at 18–22 weeks. Group B Streptococcus (GBS) testing is performed at 36–37 weeks to guide antibiotic use during labor. Missing these windows means waiting until the next pregnancy or relying on less accurate alternatives.

Beyond screening schedules, gestational age determines clinical thresholds for intervention. Fetal viability — the point at which a baby can survive outside the womb with medical support — is generally accepted at 24 weeks, with survival rates improving significantly by 28 weeks and approaching full-term rates by 34 weeks. Medical decisions about preterm labor management, steroid administration for lung maturation, and delivery timing all hinge on precise gestational age. The difference between 23 weeks 6 days and 24 weeks 0 days can determine the clinical approach to premature labor in a hospital setting.

For expectant parents, tracking pregnancy milestones provides the context to understand what is happening with fetal development at each stage. At 8 weeks, the fetus has all major organs beginning to form. At 20 weeks, the baby can hear sounds and begins REM sleep cycles. At 28 weeks, the eyes open. At 36 weeks, the lungs are mature enough for survival without intensive support in most cases. This developmental timeline — visible in the milestone output of this calculator — transforms abstract gestational age numbers into meaningful developmental context.

This calculator applies Naegele's rule, which assumes a standard 28-day cycle with ovulation on day 14. Women with longer cycles (e.g., 35–40 days) ovulate later, meaning conception occurs later and the true due date is later than Naegele's rule would calculate. Women with shorter cycles (e.g., 21–24 days) ovulate earlier. The error from assuming a standard cycle can be 1–2 weeks. For women with known irregular cycles, first-trimester ultrasound dating is significantly more accurate than LMP-based calculation.

The milestone timeline uses typical gestational age windows for standard prenatal screening and developmental events. Individual pregnancies vary, and your healthcare provider may schedule or recommend different screening timelines based on your specific health history, risk factors, geographic location, and healthcare system. Some screenings (like cell-free DNA testing for chromosomal conditions) may be offered earlier or later than the standard windows shown. Always coordinate prenatal screening schedules with your OB/GYN or midwife.

This calculator does not assess pregnancy health, risk factors, or complications. It provides gestational dating information only. Any concerns about symptoms, fetal movement, bleeding, or pain during pregnancy should be addressed with a healthcare provider immediately — not estimated from a calculator. The calculator is a planning and information tool, not a medical assessment.