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Gestational Age Calculator

Calculate gestational age and estimated due date (EDD) based on LMP. Free and accurate tool for healthcare professionals.

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Calculate Gestational Age

Enter the last menstrual period (LMP) date

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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.

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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)

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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.

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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).

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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.

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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. 1. American College of Obstetricians and Gynecologists. Methods for estimating the due date. Committee Opinion No. 700. Obstet Gynecol. 2017;129:e150-4.
  2. 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. 3. Spong CY. Defining "term" pregnancy: recommendations from the Defining "Term" Pregnancy Workgroup. JAMA. 2013;309(23):2445-2446.
  4. 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. 5. World Health Organization. WHO recommendations on antenatal care for a positive pregnancy experience. Geneva: WHO; 2016.

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.

EDD (Naegele's Rule) = LMP + 7 days - 3 months + 1 year

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.

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

What is the difference between gestational age and conceptional age?
Gestational age is counted from the first day of the last menstrual period (LMP), while conceptional age is counted from fertilization. Gestational age is approximately 2 weeks longer than conceptional age, as it includes the 2 weeks between menstruation and ovulation/fertilization. For example, 8 weeks gestation = 6 weeks since conception.
Why is first trimester ultrasound more accurate?
In the first trimester, embryonic growth is very standardized and less influenced by genetic or environmental factors. All embryos grow at the same rate in this phase. From the second trimester onwards, genetic variability (naturally larger or smaller fetuses) makes dating less accurate. Crown-rump length (CRL) between 7-13 weeks has margin of error of only ±5-7 days.
When should I correct LMP by ultrasound?
Guidelines recommend: If discrepancy >7 days in 1st trimester (7-13 weeks), >10 days in 2nd trimester (14-20 weeks), or >14 days in 3rd trimester, adopt ultrasound dating. First trimester ultrasound always supersedes LMP if there is significant discrepancy. If first ultrasound is in 2nd or 3rd trimester, consider fetal growth variability.
How to calculate EDD if my cycles are irregular?
If cycles are irregular, LMP is not reliable for calculating EDD. In this case, dating should be done exclusively by first trimester ultrasound (ideally between 11-14 weeks). If you have passed the first trimester, ultrasound can still date the pregnancy, but with less accuracy. Inform your obstetrician about irregular cycles at the first visit.
When is pregnancy considered term?
Current terminology is: Preterm: <37 weeks, Early term: 37-38 weeks and 6 days, Full term: 39-40 weeks and 6 days, Late term: 41-41 weeks and 6 days, Post-term: ≥42 weeks. Full term (39-40 weeks) is the ideal period for birth, with lowest maternal and neonatal risks. Elective cesareans should be scheduled ≥39 weeks.