Pregnancy Week Calculator
Calculate pregnancy week quickly with our pregnancy tool. Get results based on evidence-based formulas with clear explanations.
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The estimated due date is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). Current gestational week = days since LMP / 7. Trimester: 1st = weeks 1-12, 2nd = weeks 13-26, 3rd = weeks 27-40. If due date is known, LMP is back-calculated as due date minus 280 days.
Last reviewed: January 2026
Worked Examples
Example 1: LMP Date Input — Early Pregnancy
Example 2: Due Date Input — Late Pregnancy
Background & Theory
The Pregnancy Week Calculator applies the following established principles and formulas. Health and medicine calculators are grounded in validated physiological measurement methods established through decades of clinical research. Body Mass Index, or BMI, is calculated by dividing weight in kilograms by height in meters squared (kg/m²), a formula originating from Adolphe Quetelet's 19th-century statistical work and later codified by the WHO into standard classifications: underweight below 18.5, normal weight 18.5 to 24.9, overweight 25 to 29.9, and obese at 30 and above. Basal Metabolic Rate quantifies the minimum energy required to sustain life at rest. The Mifflin-St Jeor equation, published in 1990 and widely regarded as the most accurate for most adults, calculates BMR as (10 × weight in kg) + (6.25 × height in cm) − (5 × age) ± sex adjustment. The older Harris-Benedict equations, revised in 1984 by Roza and Shizgal, remain in common use. Total Daily Energy Expenditure is derived by multiplying BMR by a physical activity factor ranging from 1.2 for sedentary individuals to 1.9 for extremely active ones, following the methodology validated by doubly labeled water studies. Body fat percentage can be estimated without laboratory equipment using the U.S. Navy circumference method, which uses neck, waist, and hip measurements, or via BMI-derived equations adjusted for age and sex. The Jackson-Pollock skinfold method offers higher precision with calipers. Blood pressure classification, according to the American College of Cardiology and the 2017 ACC/AHA guidelines, defines normal as below 120/80 mmHg, elevated as 120 to 129 systolic, and hypertension stage 1 as 130 to 139 systolic or 80 to 89 diastolic. Target heart rate zones for aerobic exercise are derived from maximum heart rate estimates, most commonly using the formula 220 minus age in years, with moderate-intensity training typically defined as 50 to 70 percent of maximum heart rate and vigorous intensity at 70 to 85 percent, consistent with CDC and American Heart Association guidelines. These thresholds guide safe and effective cardiovascular conditioning.
History
The history behind the Pregnancy Week Calculator traces back through the following developments. The history of health measurement stretches back to ancient Greece, where Hippocrates around 400 BCE laid the foundation for observational medicine by systematically recording patient symptoms, diet, and environment. His humoral theory, though scientifically superseded, established the principle that the body operates as an interconnected system subject to measurable imbalance. The transformation toward modern medicine accelerated in the 19th century. Louis Pasteur and Robert Koch developed germ theory in the 1860s and 1870s, identifying microorganisms as disease agents and enabling targeted interventions. Florence Nightingale, working during the Crimean War in the 1850s, introduced statistical analysis to nursing practice, demonstrating through data visualization that sanitation reduced mortality. Her work is foundational to evidence-based health measurement. The discovery of vitamins in the early 20th century, beginning with Casimir Funk's coinage of the term in 1912 and culminating in the isolation of vitamins A through K, created the field of nutritional science and gave rise to dietary reference intake frameworks. The World Health Organization, founded in 1948, subsequently established global standards for health metrics, disease classification through the International Classification of Diseases, and recommended daily allowances. The BMI as a clinical screening tool gained traction in the 1970s through Ancel Keys' large-scale epidemiological work, which validated Quetelet's index as a population-level obesity indicator. Through the 1980s and 1990s, the Framingham Heart Study produced landmark data linking cholesterol, blood pressure, and lifestyle factors to cardiovascular disease risk, directly shaping the numeric thresholds still used in health calculators. The evidence-based medicine movement, formalized by Gordon Guyatt and colleagues at McMaster University in the early 1990s, demanded that all health recommendations derive from systematically graded clinical evidence. The digital health era beginning in the 2000s brought these formulas to consumer devices, wearable sensors, and smartphone applications, expanding access to health self-monitoring on a global scale and enabling population-level data collection that continues to refine clinical reference ranges.
Frequently Asked Questions
Formula
Due Date = LMP + 280 days (Naegele's Rule)
The estimated due date is calculated by adding 280 days (40 weeks) to the first day of the last menstrual period (LMP). Current gestational week = days since LMP / 7. Trimester: 1st = weeks 1-12, 2nd = weeks 13-26, 3rd = weeks 27-40. If due date is known, LMP is back-calculated as due date minus 280 days.
Worked Examples
Example 1: LMP Date Input — Early Pregnancy
Problem: Last menstrual period was January 15, 2026. What is the current pregnancy status? (Assume today is March 21, 2026.)
Solution: LMP: January 15, 2026\nDue date (LMP + 280 days): October 22, 2026\nDays since LMP: 65 days\nCurrent week: 65 / 7 = Week 9, Day 2\nTrimester: 1st (weeks 1-12)\nDays remaining: 280 - 65 = 215 days\nBaby size comparison: Cherry
Result: Week 9, Day 2 | 1st Trimester | Due: October 22, 2026 | 215 days remaining
Example 2: Due Date Input — Late Pregnancy
Problem: Due date is April 15, 2026. What is the current status? (Assume today is March 21, 2026.)
Solution: Due date: April 15, 2026\nLMP (due date - 280 days): July 9, 2025\nDays since LMP: 256 days\nCurrent week: 256 / 7 = Week 36, Day 4\nTrimester: 3rd (weeks 27-40)\nDays remaining: 280 - 256 = 24 days\nBaby size comparison: Romaine lettuce
Result: Week 36, Day 4 | 3rd Trimester | Due: April 15, 2026 | 24 days remaining
Frequently Asked Questions
How is pregnancy due date calculated?
Pregnancy due date is most commonly calculated using Naegele's rule, which adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). This assumes a 28-day menstrual cycle with ovulation occurring on day 14. The formula was developed by German obstetrician Franz Naegele in the early 1800s and remains the standard method used by healthcare providers today. However, only about 4% of babies are born on their exact due date. Most babies arrive within two weeks before or after the estimated date. Ultrasound dating in the first trimester provides more accurate dating, especially for women with irregular cycles, and may adjust the estimated due date by several days.
What are the three trimesters of pregnancy?
Pregnancy is divided into three trimesters, each approximately 13 weeks long. The first trimester (weeks 1-12) is the period of rapid organ development when all major body systems form. This is when morning sickness is most common and the risk of miscarriage is highest. The second trimester (weeks 13-26) is often called the golden period because many uncomfortable symptoms subside, energy increases, and the baby bump becomes visible. Fetal movements (quickening) are typically first felt around week 20. The third trimester (weeks 27-40) involves rapid fetal growth and brain development. The baby gains most of its weight during this period, and the mother may experience increased fatigue, back pain, and Braxton Hicks contractions.
When should I first see a doctor during pregnancy?
Most healthcare providers recommend scheduling your first prenatal visit between weeks 6 and 8 of pregnancy, counted from the first day of your last menstrual period. At this visit, the provider confirms the pregnancy with blood work and potentially an early ultrasound, establishes the due date, reviews medical history, orders baseline lab work, and discusses prenatal vitamins. However, you should contact your provider immediately if you experience severe abdominal pain, heavy bleeding, high fever, or severe vomiting causing dehydration. After the initial visit, prenatal appointments are typically monthly until week 28, every two weeks until week 36, then weekly until delivery. More frequent monitoring may be recommended for high-risk pregnancies.
How accurate are the results from Pregnancy Week Calculator?
All calculations use established mathematical formulas and are performed with high-precision arithmetic. Results are accurate to the precision shown. For critical decisions in finance, medicine, or engineering, always verify results with a qualified professional.
How do I get the most accurate result?
Enter values as precisely as possible using the correct units for each field. Check that you have selected the right unit (e.g. kilograms vs pounds, meters vs feet) before calculating. Rounding inputs early can reduce output precision.
Does Pregnancy Week Calculator work offline?
Once the page is loaded, the calculation logic runs entirely in your browser. If you have already opened the page, most calculators will continue to work even if your internet connection is lost, since no server requests are needed for computation.
References
Reviewed by Rahul Singh, Health & Wellness Specialist · Editorial policy