--- title: "Due Date Calculator" site: ProCalc.ai section: Health url: https://procalc.ai/health/due-date-calculator markdown_url: https://procalc.ai/health/due-date-calculator.md date_published: 2026-04-11 date_modified: 2026-04-13 date_created: 2026-03-15 input_mode: focused --- # Due Date Calculator **Site:** [ProCalc.ai](https://procalc.ai) — Free Professional Calculators **Section:** Health **Calculator URL:** https://procalc.ai/health/due-date-calculator **Markdown URL:** https://procalc.ai/health/due-date-calculator.md **Published:** 2026-04-11 **Last Updated:** 2026-04-13 **Description:** Free Due Date Calculator — Calculate your estimated due date based on your last menstrual period or conception date. See your pregnancy timeline ... > *This file is served for AI systems and search crawlers. Human page: https://procalc.ai/health/due-date-calculator* ## Overview Planning around a pregnancy is easier when you can see the dates laid out clearly, and ProCalc.ai’s Due Date Calculator helps you do exactly that. You use the Due Date Calculator to estimate your due date and map out your pregnancy timeline, including trimester start and end dates, so you can keep appointments, work plans, and personal milestones in sync. It’s commonly used by people who’ve just had a positive test and want a realistic timeline to share with their OB-GYN or midwife at the first visit. For example, if you’re scheduling your first prenatal appointment and trying to figure out… ## Formula This due date calculator uses standard health formulas to compute results. Enter your values and the formula is applied automatically — all math is handled for you. The calculation follows industry-standard methodology. ## How to Use ## How Pregnancy Due Dates Are Calculated A **due date** (estimated date of delivery, or EDD) is calculated by adding **280 days (40 weeks)** to the first day of the last menstrual period (LMP). This is known as Naegele's rule, developed in the early 19th century and still the standard method used by obstetricians worldwide. The 40-week calculation assumes a 28-day cycle with ovulation occurring on day 14. ## Using the Calculator Enter the first day of your last menstrual period and your average cycle length. The calculator shows your estimated due date, current gestational age in weeks and days, trimester, and key milestone dates throughout pregnancy. If your cycle is longer or shorter than 28 days, the calculator adjusts the due date accordingly. ## Trimester Milestones **First trimester (weeks 1-12)**: Major organ formation occurs. First prenatal visit typically at 8-10 weeks. First ultrasound confirms viability and may adjust the due date. **Second trimester (weeks 13-27)**: Anatomy scan at 18-22 weeks. Fetal movement felt around 16-25 weeks. Glucose screening at 24-28 weeks. **Third trimester (weeks 28-40)**: Growth monitoring, Group B strep test at 36 weeks, weekly visits from 36 weeks onward. ## Due Date Accuracy Only about **5% of babies** are born on their exact due date. The due date represents the midpoint of a normal delivery range — full term is defined as **39 weeks 0 days through 40 weeks 6 days** by the American College of Obstetricians and Gynecologists. Deliveries between 37 and 42 weeks are considered within the normal range. First pregnancies tend to run slightly longer on average. ## Ultrasound Dating vs LMP Dating A first-trimester ultrasound measuring **crown-rump length** is the most accurate method for determining gestational age — accurate to within ±5-7 days. If the ultrasound date differs from the LMP-calculated date by more than 7 days, the ultrasound date is generally preferred. Later ultrasounds are less precise for dating (±2-3 weeks in the third trimester). ## Cycle Length Adjustments Naegele's rule assumes a 28-day cycle. For longer cycles, ovulation occurs later, pushing the due date forward. A woman with a 35-day cycle likely ovulated around day 21 instead of day 14 — adding 7 days to the standard due date. The calculator makes this adjustment automatically when you enter your average cycle length. ## Authoritative Sources This calculator uses formulas and reference data drawn from the following sources: - [CDC — Physical Activity](https://www.cdc.gov/physical-activity/) - [NIH — National Institute of Diabetes](https://www.niddk.nih.gov/) - [NIH — National Heart, Lung, and Blood Institute](https://www.nhlbi.nih.gov/) ## Frequently Asked Questions ### How is the due date calculated? The most common method (Naegele's Rule) adds 280 days (40 weeks) to the first day of your last menstrual period. If calculating from conception, it adds 266 days (38 weeks). ### How accurate is a due date calculator? Due date calculators provide an estimate. Only about 5% of babies are born on their exact due date. Most arrive within a two-week window around the estimated date. Your healthcare provider may adjust the date based on ultrasound measurements. ### What's the difference between LMP and conception date? LMP (Last Menstrual Period) is the first day of your last period — the standard medical dating method. Conception typically occurs about 14 days after LMP, during ovulation. If you know your conception date, the calculator adjusts accordingly. ### What are the three trimesters? The first trimester covers weeks 1-12, the second trimester weeks 13-26, and the third trimester weeks 27-40. Each trimester brings different developmental milestones and changes. ### Can my due date change? Yes. Early ultrasounds (especially before 12 weeks) can adjust the due date if measurements differ from LMP-based calculations by more than a week. Your doctor will determine the most accurate estimate. ### How does the Due Date Calculator work? The calculator estimates your due date by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP), which is the standard Naegele’s rule. If you enter a known conception date, it estimates due date by adding about 266 days (38 weeks). Results assume a typical cycle and may be adjusted by your clinician based on ultrasound dating. ### Conception date vs implantation date — what's the difference? Conception (fertilization) is when sperm meets egg, typically around ovulation. Implantation happens later, when the embryo attaches to the uterine lining, usually about 6–10 days after fertilization. Pregnancy tests generally turn positive after implantation because hCG rises once implantation occurs. ### Can I use this for IVF or IUI pregnancies? Yes, but the most accurate input is the procedure-related date rather than LMP. For IVF, clinics often date pregnancy using the egg retrieval or embryo transfer date plus the embryo’s age (e.g., day-3 or day-5 transfer). For IUI, using the insemination date or ovulation trigger date can be more reliable than LMP, especially with irregular cycles. ## Sources - [NIH](https://www.nih.gov) - [WHO](https://www.who.int) - [CDC](https://www.cdc.gov) --- ## Reference - **Calculator page:** https://procalc.ai/health/due-date-calculator - **This markdown file:** https://procalc.ai/health/due-date-calculator.md ### AI & Developer Resources - **LLM index (short):** https://procalc.ai/llms.txt - **LLM index (full, with content):** https://procalc.ai/llms-full.txt - **MCP server:** https://procalc.ai/api/mcp - **Materials JSON API:** https://procalc.ai/api/materials.json - **Developer docs:** https://procalc.ai/developers - **Sitemap:** https://procalc.ai/sitemap.xml - **Robots:** https://procalc.ai/robots.txt ### How to Cite > ProCalc.ai. "Due Date Calculator." ProCalc.ai, 2026-04-11. https://procalc.ai/health/due-date-calculator ### License Content © ProCalc.ai. Free to reference and cite. 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