Weeks of Gestation Calculator
The Weeks of Gestation Calculator estimates the current gestational age of a pregnancy based on the First Day of the Last Menstrual Period (LMP). This calculator is designed to help healthcare providers and individuals monitoring pregnancy progression explore estimated fetal age and due date. Whether you're tracking pregnancy milestones, preparing for prenatal appointments, or estimating your due date, this tool provides calculated values based on standard obstetric methods.
This calculator provides estimates based on general averages. Individual results may vary. Consult a healthcare provider for personalized guidance.
How This Calculator Works
- Enter the first day of your last menstrual period (LMP) in the date field.
- Confirm or adjust the current date for the calculation.
- Click "Calculate" to compute your estimated gestational age and due date.
- Review your estimated values, including weeks, days, and trimester information.
How Gestational Age Is Calculated
Gestational age represents the duration of a pregnancy measured from the first day of the last menstrual period (LMP). This method, widely used in obstetrics, provides a standardized way to estimate fetal development and timeline. The calculation counts forward from the LMP date rather than from conception, which typically occurs approximately two weeks after the LMP in a regular 28-day cycle.
Total Days = Current Date − LMP Date
Weeks = floor(Total Days ÷ 7)
Days = Total Days mod 7
Where:
- LMP Date = First day of the last menstrual period
- Current Date = The date for which gestational age is calculated
- floor = Function that rounds down to the nearest whole number
- mod = Modulo operation giving the remainder after division
The Estimated Due Date (EDD) is calculated using Naegele's Rule, which adds 280 days (40 weeks) to the LMP date. This rule assumes a 28-day menstrual cycle with ovulation occurring around day 14. For individuals with irregular cycles or uncertain LMP dates, ultrasound measurements may provide more accurate dating.
These calculations provide estimates based on standard obstetric practices. Actual gestational age and due dates may vary based on individual factors such as cycle length and ovulation timing.
What Your Gestational Age Means
Your gestational age represents the estimated time elapsed since the first day of your last menstrual period. Healthcare providers use this measurement to track pregnancy milestones, schedule prenatal screenings, and monitor fetal development. The calculation helps establish a timeline for expected growth patterns and important medical checkups throughout pregnancy.
| Trimester | Gestational Age | Key Developments |
|---|---|---|
| First Trimester | 0 – 13 weeks 6 days | Organ formation, heartbeat detectable |
| Second Trimester | 14 – 27 weeks 6 days | Movement felt, detailed anatomy scan |
| Third Trimester | 28 – 40+ weeks | Rapid growth, lung maturation |
For Early Pregnancy (Under 12 Weeks): This period may involve confirming the pregnancy with a healthcare provider, scheduling initial prenatal bloodwork, and discussing early prenatal care options. First trimester screenings for genetic conditions are typically offered between 10-13 weeks.
For Mid-Pregnancy (12-28 Weeks): Many individuals schedule an anatomy ultrasound around 18-22 weeks. This period often includes glucose screening tests around 24-28 weeks and discussions about birthing preferences with healthcare providers.
For Late Pregnancy (28+ Weeks): Prenatal visits typically become more frequent during this time. Discussions with healthcare providers about labor signs, birth plans, and postpartum preparation may be helpful. Regular monitoring of fetal movement is often recommended.
Important: This calculator uses the LMP method, which may differ from dating based on ultrasound measurements. Ultrasound dating, particularly in the first trimester, is often considered more accurate for pregnancies where LMP is uncertain or cycles are irregular. Discuss with a healthcare provider which dating method is most appropriate for your situation.
Regular prenatal care allows healthcare providers to monitor both fetal development and maternal health throughout pregnancy. Keeping track of gestational age helps ensure timely screenings and appropriate medical support at each stage.
References
- American College of Obstetricians and Gynecologists (ACOG) – Methods for Estimating the Due Date
- American Academy of Family Physicians (AAFP) – Prenatal Care and Gestational Age Assessment
- March of Dimes – Pregnancy Week by Week Development Timeline
- National Institutes of Health (NIH) – Naegele's Rule and Pregnancy Dating Methods
Calculation logic verified using publicly available standards.
View our Accuracy & Reliability Framework →