Important Medical Disclaimer
This calculator is an educational and clinical decision support tool. Results DO NOT replace professional medical evaluation, laboratory tests, or clinical judgment. Always consult a qualified healthcare professional for diagnosis, treatment, and clinical decisions. Calculations are based on scientifically validated formulas but may not be applicable to all patients.
Gestational Age Calculator
Calculate gestational age and estimated due date based on Last Menstrual Period (LMP).
Calculate Gestational Age
Enter the last menstrual period (LMP) date
What is Gestational Age?
Gestational age is the time elapsed from the first day of the last menstrual period (LMP) to the current date, expressed in weeks and days. It is fundamental for prenatal care, determining the estimated due date (EDD), assessing fetal growth, and planning obstetric interventions. Gestational age differs from conceptional age (time since fertilization), being approximately 2 weeks longer, as it considers the complete menstrual cycle.
How to Calculate Gestational Age?
Gestational age is calculated from the LMP (Last Menstrual Period). To use this calculator: 1) Enter the first day of the last menstrual period, 2) Click 'Calculate'. The result will show gestational age in weeks and days, EDD (Estimated Due Date) calculated by Naegele's Rule (LMP + 280 days or 40 weeks), and current trimester. For example, if LMP was 12 weeks and 3 days ago, the pregnancy is 12 weeks and 3 days, at the end of the 1st trimester.
Formula (Naegele's Rule):
EDD = LMP + 280 days (40 weeks)
Pregnancy Trimesters
1st Trimester
0-13 weeks
Period of organogenesis. Higher risk of malformations from teratogens. Common nausea, vomiting, and fatigue. First trimester ultrasound (11-14 weeks) is most accurate for pregnancy dating.
2nd Trimester
14-27 weeks
Period of accelerated fetal growth. Reduction of 1st trimester symptoms. Fetal movements perceptible (quickening) from 18-20 weeks. Morphological ultrasound between 20-24 weeks.
3rd Trimester
28-40+ weeks
Final fetal maturation. Accelerated weight gain. Preparation for delivery. More frequent monitoring. Early term: 37-38 weeks, Full term: 39-40 weeks, Late term: 41 weeks, Post-term: ≥42 weeks.
Clinical Use of Gestational Age
Prenatal Care Schedule
Defines the schedule for consultations, laboratory tests, and ultrasounds. First visit ideally in the 1st trimester, with monthly visits until 28 weeks, biweekly until 36 weeks, and weekly until delivery.
Screening and Diagnosis
Accurate gestational age is essential to interpret biochemical markers (PAPP-A, β-hCG) and ultrasound markers (nuchal translucency) in aneuploidy screening.
Timing of Interventions
Determines optimal timing for procedures such as amniocentese (15-20 weeks), antenatal corticosteroids for lung maturation (24-34 weeks), and elective cesarean section planning (≥39 weeks).
Limitations and Important Considerations
LMP is reliable only in women with regular 28-day menstrual cycles. With irregular cycles, anovulation, recent birth control pill use, or breastfeeding, LMP may be inaccurate. Implantation bleeding may be confused with menstruation, leading to dating errors. First trimester ultrasound (7-13 weeks) is the gold standard for dating, with margin of error of only ±5-7 days. If there is discrepancy >7 days between LMP and first trimester ultrasound, ultrasound dating should be adopted. In vitro fertilization (IVF) pregnancies have exact dating by embryonic age.
Frequently Asked Questions
Related Topics
- Naegele's Rule for EDD calculation
- Obstetric ultrasound for dating
- Crown-rump length (CRL) in 1st trimester
- Aneuploidy screening in prenatal care
- Prematurity and term classification
- Fetal biometry and growth curves
Scientific References
- 1. American College of Obstetricians and Gynecologists. Methods for estimating the due date. Committee Opinion No. 700. Obstet Gynecol. 2017;129:e150-4.
- 2. Jukic AM, Baird DD, Weinberg CR, McConnaughey DR, Wilcox AJ. Length of human pregnancy and contributors to its natural variation. Hum Reprod. 2013;28(10):2848-2855.
- 3. Spong CY. Defining "term" pregnancy: recommendations from the Defining "Term" Pregnancy Workgroup. JAMA. 2013;309(23):2445-2446.
- 4. Napolitano R, Dhami J, Ohuma EO, et al. Pregnancy dating by fetal crown-rump length: a systematic review of charts. BJOG. 2014;121(5):556-565.
- 5. World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience. Geneva: WHO; 2016.