Pregnancy Calculator — Due Date & Gestational Tracker

The first trimester spans from LMP through week 13. During these critical weeks, single-cell zygotes transform into complex fetuses with beating hearts (detectable by transvaginal ultrasound by ~6 weeks), forming neural tubes, developing limb buds, and establishing early placental circulation. Critical prenatal screenings include nuchal translucency (NT) scans and cell-free DNA (NIPT).

Spanning weeks 14 through 27, the second trimester often brings relief from early nausea. Expectant mothers typically perceive fetal movements (“quickening”) between weeks 18 and 22. The detailed fetal anatomy ultrasound (level II scan) evaluates organ structures, spine alignment, and facial profiles, while 24 weeks marks the clinical milestone of potential viability outside the womb.

From week 28 until birth, the fetus gains substantial adipose tissue, practices respiratory chest wall motions, and matures alveolar lung surfactants. “Full term” is officially achieved at 39 weeks 0 days. Regular prenatal visits increase in frequency to monitor blood pressure, fundal height, fetal position, and cervical ripening.

How is gestational age calculated differently from conception age?

Gestational age is calculated from the first day of your last menstrual period (LMP), while conception (embryonic) age begins at fertilization, typically two weeks later in a 28-day cycle. Because fertilization time is rarely known with absolute certainty, obstetricians universally standardize pregnancy dating to gestational age (40 weeks / 280 days).

What is Naegele’s Rule and how does it calculate my due date?

Naegele’s Rule is the global clinical baseline for estimated date of delivery (EDD). It takes the first day of your last menstrual period, adds 1 year, subtracts 3 months, and adds 7 days (or simply adds 280 days). Our calculator further adjusts this calculation if your typical menstrual cycle is shorter or longer than the standard 28-day cycle.

What happens if my menstrual cycle is longer or shorter than 28 days?

Menstrual cycle length variations primarily affect the follicular phase before ovulation, while the luteal phase remains relatively stable at 14 days. If your cycle is 35 days, you likely ovulated around day 21 (7 days later than average), shifting your true due date 7 days later than standard LMP charts indicate. Our tool automatically reconciles this cycle offset.

How accurate is an estimated due date (EDD)?

Only 4% to 5% of babies arrive on their exact calculated due date. However, roughly 68% deliver within 10 days of the EDD, and over 80% deliver during the full-term window between 37 weeks 0 days and 41 weeks 6 days. Due dates should be understood as an estimated median of a biological window rather than a strict deadline.

When does an ultrasound scan override an LMP due date?

Under American College of Obstetricians and Gynecologists (ACOG) Committee Opinion No. 700 guidelines, first-trimester crown-rump length (CRL) ultrasound measurements are the gold standard. If an early ultrasound (before 9 weeks) differs from your LMP due date by more than 5 days, or by more than 7 days between weeks 9 and 14, the ultrasound estimate replaces the LMP date.

Is my personal reproductive and pregnancy data saved on this website?

No. Pregnancy Calculator is built with a client-side privacy architecture. All mathematical calculations execute exclusively inside your web browser’s local memory. No period dates, conception estimates, or personal health metrics are transmitted, saved on our servers, or shared with third-party tracking networks.