Pregnancy Calculator
Track your pregnancy journey with our due date calculator. Enter your last menstrual period to see your estimated due date, current week, trimester, and key milestones.
Key features
- Estimated due date calculation
- Current week and trimester tracking
- Conception date estimation
- Key pregnancy milestones timeline
Guide
Tracking a pregnancy timeline involves calculating an estimated due date, identifying the current trimester, and understanding what developmental milestones happen each week. A pregnancy due date calculator takes a starting reference point, usually the first day of the last menstrual period (LMP), and projects forward to estimate the date of delivery. This guide covers the math behind due date calculation, what each trimester involves, how gestational age differs from fetal age, and how to use weekly tracking data for better prenatal planning. The standard pregnancy calculation assumes a 280-day (40-week) gestation measured from the first day of the last menstrual period. This method is called Naegele's rule, named after the German obstetrician who formalized it in the early 1800s. The formula adds 7 days to the LMP date and then adds 9 calendar months, or equivalently subtracts 3 months and adds 1 year and 7 days. For example, if the LMP was January 1, adding 7 days gives January 8, subtracting 3 months gives October 8 of the same year, and that is the estimated due date. The WebRecast pregnancy calculator automates this arithmetic and also accounts for different cycle lengths. The 280-day assumption is based on a 28-day menstrual cycle with ovulation occurring on day 14. Many people do not have a 28-day cycle. If your cycle is regularly 32 days, ovulation likely occurs around day 18, which means conception happens 4 days later than the standard model assumes. A good pregnancy calculator lets you input your average cycle length and adjusts the due date accordingly. For a 32-day cycle, the estimated due date shifts 4 days later compared to the standard calculation. For a 24-day cycle, it shifts 4 days earlier. Gestational age and fetal age are different measurements that cause frequent confusion. Gestational age counts from the first day of the last menstrual period, which is roughly 2 weeks before conception actually occurs. Fetal age (also called embryonic age or conceptional age) counts from the estimated date of fertilization. When a doctor says you are 8 weeks pregnant, that is gestational age, and the embryo is approximately 6 weeks old. Medical professionals and pregnancy resources almost always use gestational age, so the calculator follows that convention. The first trimester spans weeks 1 through 12 of gestational age. During weeks 1 and 2, the body prepares for ovulation and conception has not yet occurred. Implantation of the fertilized egg typically happens around week 4. By week 5, the embryonic heart begins to form and starts beating. Week 6 brings early brain development and limb buds. By week 8, all major organs have begun forming, and the embryo is officially called a fetus. Weeks 9 through 12 see rapid growth, with fingernails forming, the face becoming more defined, and the fetus reaching about 2.5 inches in length by the end of week 12. Morning sickness, fatigue, and breast tenderness are common first-trimester symptoms that peak between weeks 8 and 12 for many people. Knowing your exact week helps contextualize these symptoms. If the calculator shows you are at week 9 and you are experiencing intense nausea, that aligns with typical timing. First-trimester screening tests, including blood work and nuchal translucency ultrasound, are performed between weeks 11 and 14. Having your gestational week clearly tracked helps you schedule these appointments on time. The second trimester covers weeks 13 through 26. This period is often called the most comfortable trimester because first-trimester nausea typically subsides and the physical demands of late pregnancy have not yet started. The anatomy scan ultrasound, one of the most detailed prenatal imaging exams, happens between weeks 18 and 22. Fetal movement (quickening) is usually first felt between weeks 16 and 22. By week 20, the fetus is about 10 inches long and weighs around 10 ounces. The glucose screening test for gestational diabetes occurs between weeks 24 and 28. The third trimester runs from week 27 through delivery, typically around week 40. The fetus gains most of its birth weight during this period, growing from roughly 2 pounds at week 27 to an average of 7 to 8 pounds at term. Lung maturation is a key development during weeks 28 through 36. The fetus shifts into a head-down position in preparation for delivery, usually between weeks 32 and 36. Group B streptococcus (GBS) testing occurs between weeks 36 and 37. Weekly prenatal visits typically begin at week 36. A due date is an estimate, not a deadline. Only about 5% of babies are born on their exact due date. The majority of births occur within a window of two weeks before to two weeks after the estimated date. Full term is defined as 39 weeks 0 days through 40 weeks 6 days. Early term is 37 weeks 0 days through 38 weeks 6 days. Late term is 41 weeks 0 days through 41 weeks 6 days. Post-term begins at 42 weeks 0 days. Understanding these definitions helps you interpret your calculator results with appropriate expectations. Ultrasound dating provides a more accurate due date than LMP-based calculation in certain situations. If the LMP is uncertain, if cycles are irregular, or if early ultrasound measurements differ from LMP-based estimates by more than 5 to 7 days, practitioners typically adjust the due date to match the ultrasound. First-trimester ultrasound measurements (crown-rump length) are the most accurate, with a margin of error of plus or minus 5 to 7 days. Second-trimester measurements have a margin of plus or minus 10 to 14 days. If your provider has adjusted your due date based on ultrasound, use the revised date in the calculator for accurate week-by-week tracking. Week-by-week tracking serves practical purposes beyond curiosity. Many prenatal tests have specific gestational age windows. The first-trimester combined screening must happen between weeks 11 and 14. Amniocentesis, if indicated, is typically performed between weeks 15 and 20. The anatomy scan happens between weeks 18 and 22. Rhogam injections for Rh-negative mothers are given around week 28. Knowing your current week ensures you do not miss time-sensitive appointments. Pregnancy weight gain guidelines depend on trimester and pre-pregnancy BMI. The general recommendation for someone with a normal pre-pregnancy BMI (18.5 to 24.9) is 25 to 35 pounds total, with most gain occurring in the second and third trimesters. First-trimester weight gain is typically 1 to 4 pounds total. Second and third trimester gains average about 1 pound per week. Tracking your current week alongside weight gain helps you and your healthcare provider assess whether gain is on track. Fetal viability is a milestone that occurs around weeks 23 to 24. Before this point, the lungs and other organs are too immature to support life outside the womb even with medical intervention. After 24 weeks, survival rates with intensive neonatal care increase significantly each week. At 24 weeks, survival rates in well-equipped NICUs are roughly 40 to 70%. By 28 weeks, survival exceeds 90%. By 34 weeks, most babies do well with minimal intervention. These statistics underscore why accurate gestational age tracking matters. Multiple pregnancies (twins, triplets) follow a different timeline. Twins are typically delivered between 36 and 38 weeks, and triplets between 32 and 34 weeks. Growth patterns, weight gain recommendations, and screening schedules differ from singleton pregnancies. If you are carrying multiples, the due date from a standard calculator serves as a rough reference, but your care team will set a more specific delivery target based on ultrasound monitoring and your specific situation. Preterm labor warning signs include regular contractions before 37 weeks, lower back pain that does not resolve, pelvic pressure, changes in vaginal discharge, and fluid leakage. Knowing your gestational week is essential context for evaluating these symptoms. Contractions at 35 weeks require different medical responses than contractions at 29 weeks. The calculator keeps your current gestational age visible so you always have this context available. Maternity leave and workplace planning benefit from accurate due date tracking. Many employers require notice 4 to 6 weeks before the expected leave date. Hospital pre-registration, birth plan preparation, and childcare arrangements all operate on timelines anchored to the due date. The calculator gives you a fixed reference point for scheduling these logistics. The countdown display in the calculator shows days remaining until the estimated due date, current gestational week and day, the trimester you are in, and how far through the pregnancy you have progressed as a percentage. This information helps you quickly communicate your status to healthcare providers, family members, and employers without needing to manually calculate from your LMP each time. Nutrition needs change by trimester. First-trimester caloric needs are roughly the same as pre-pregnancy levels. Second-trimester needs increase by about 340 calories per day. Third-trimester needs increase by about 450 calories per day. Folate is critical in the first trimester for neural tube development. Iron needs increase throughout pregnancy, peaking in the third trimester. Calcium and vitamin D requirements remain elevated throughout all three trimesters. Tracking your current trimester helps you adjust your nutrition focus appropriately. Exercise guidelines also vary by trimester and individual health status. Most people can continue moderate exercise throughout pregnancy with modifications. First-trimester exercise may need to accommodate nausea and fatigue. Second-trimester adjustments include avoiding supine exercises after week 20 (lying flat on the back can compress the vena cava) and reducing high-impact activities as the center of gravity shifts. Third-trimester exercise focuses on maintaining mobility and preparing the body for labor. Walking, swimming, prenatal yoga, and modified strength training are commonly recommended throughout pregnancy. The pregnancy calculator stores no personal health data on external servers. All calculations happen in your browser. This privacy consideration matters because pregnancy status is sensitive personal information that many people prefer to control carefully, especially in the first trimester before announcing to family and friends. Using the calculator is straightforward. Enter the first day of your last menstrual period. Optionally adjust the cycle length if yours differs from 28 days. The tool calculates and displays your estimated due date, current gestational age in weeks and days, current trimester, and a week-by-week reference of key developmental milestones. Bookmark the page or save your LMP date so you can check back anytime for updated progress information as your pregnancy advances.
Frequently asked questions
How is the due date calculated?
The due date is estimated by adding 280 days (40 weeks) to the first day of your last menstrual period (LMP).
How accurate is the due date?
Only about 5% of babies are born on their exact due date. Most are born within 2 weeks before or after.
