Evidence-Based Obstetric Formula

Pregnancy Calculator

Calculate your estimated due date, current gestational age in weeks and days, and trimester timeline using standard clinical algorithms based on your last menstrual period or date of conception.

ACOG & Naegele's Method Updated Sep 2026

Interactive Pregnancy Calculator

Estimate your due date, current gestational weeks and days, and track your trimester progress.

Range: 20–45 days

Adjusting cycle length customizes the estimated ovulation date by adding or subtracting days from standard 28-day assumptions.

How Does the Pregnancy Calculator Work?

In clinical obstetrics, pregnancy duration is conventionally measured from the first day of your last menstrual period (LMP) rather than the exact moment of fertilization. This standard convention—known as gestational age—adds approximately two weeks before ovulation occurs.

Our calculator applies Naegele's Rule (adding 280 days / 40 completed weeks to your LMP) and allows you to calibrate for menstrual cycle length variations. If you know your approximate conception or ovulation date, the calculator deterministically adds 266 days (38 post-fertilization weeks).

1. LMP Method

Adds 280 days to the first day of your last period, adjusting if your cycle is shorter or longer than the standard 28 days.

2. Conception Method

Adds 266 days (38 weeks) from the estimated date of fertilization, representing biological embryonic age.

3. Pregnancy Now Mode

Compares your reference dates with today's live calendar date to show your exact current week, day, and trimester.

Understanding Your Pregnancy Trimesters

First Trimester (Weeks 1–13)Foundational

From cellular division and implantation to vital organogenesis. By week 10, all essential bodily structures and limbs are established, transitioning the embryo into a fetus.

Second Trimester (Weeks 14–27)Anatomy & Growth

Marked by rapid anatomical maturation and detectable maternal movement (quickening). The mid-pregnancy anatomy ultrasound typically takes place around week 18–22.

Third Trimester (Weeks 28–40+)Maturation

Critical lung surfactant development, brain synaptogenesis, and rapid fetal fat accumulation preparing for birth. Full term is achieved at 39 completed weeks.

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