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Gestational Age Calculator

Calculate your baby's gestational age, due date, and important pregnancy milestones based on your last menstrual period or conception date.

Date Information

How to Use This Calculator

For the most accurate results, follow these guidelines:

Last Menstrual Period (LMP):

  • Enter the first day of your last menstrual period
  • Specify your average menstrual cycle length (default is 28 days)
  • The standard calculation adds 280 days (40 weeks) to the LMP

Conception Date:

  • Use if you know your conception date (e.g., from IVF or fertility tracking)
  • The calculation adds 266 days (38 weeks) to conception date

If you've had an ultrasound that estimated gestational age, enter that information for a more accurate calculation.

Current Gestational Age

0 weeks, 0 days
First Trimester Second Trimester Third Trimester

Your baby is developing rapidly at this stage. The current trimester is characterized by significant developmental milestones.

Important Dates

Due Date (EDD): --

First Trimester: --

Second Trimester: --

Third Trimester: --

Conception Date (est.): --

Fetal Viability (24 weeks): --

Full Term (37 weeks): --

Key Development Milestones

What is Gestational Age?
How It's Calculated
Understanding Trimesters
Developmental Milestones
Ultrasound Dating

What is Gestational Age?

Gestational age refers to how far along a pregnancy is, measured from the first day of the mother's last menstrual period (LMP) rather than from the actual date of conception. This is the standard clinical method used by healthcare providers worldwide.

While this may seem counterintuitive since fertilization typically occurs about two weeks after the LMP, this convention was established because:

  • The exact date of conception is often unknown
  • The LMP is typically easier for women to recall accurately
  • This method provides a consistent starting point for all pregnancies

By this standard method, a full-term pregnancy lasts approximately 40 weeks (280 days) from the LMP, which is equivalent to 38 weeks (266 days) from conception.

Gestational age is commonly expressed in weeks and days, such as "28 weeks and 5 days" (sometimes written as 28+5).

How Gestational Age is Calculated

There are several methods used to calculate gestational age:

  • Naegele's Rule (LMP Method): The most common calculation adds 280 days (40 weeks) to the first day of the last menstrual period. This method assumes a 28-day menstrual cycle with ovulation occurring on day 14.
  • Adjusted LMP Method: For women with irregular cycles, the due date can be adjusted. If your cycle is longer than 28 days, add the additional days to the due date; if shorter, subtract the days.
  • Conception Date Method: If the conception date is known (as with IVF or careful fertility tracking), add 266 days (38 weeks) to that date.
  • Ultrasound Dating: Early ultrasounds (especially first trimester) can provide accurate dating by measuring the embryo or fetus. These measurements are compared to standard growth charts to determine gestational age.

Early pregnancy ultrasounds are considered the most accurate method for dating a pregnancy, especially when performed in the first trimester. If there's a significant discrepancy between LMP dating and ultrasound dating (more than 5-7 days in the first trimester), healthcare providers typically defer to the ultrasound date.

Understanding Pregnancy Trimesters

A pregnancy is typically divided into three trimesters, each lasting approximately 13-14 weeks:

  • First Trimester (Weeks 1-12): This critical period involves rapid development of the embryo, with all major organs forming. Common symptoms include morning sickness, fatigue, breast tenderness, and frequent urination. Risk of miscarriage is highest during this period.
  • Second Trimester (Weeks 13-26): Often called the "golden period" of pregnancy. Symptoms like nausea typically improve, energy levels increase, and the baby's movements become noticeable. The baby's gender can usually be determined during this period. The baby develops facial features, and organs continue to mature.
  • Third Trimester (Weeks 27-40): The baby grows rapidly and gains weight in preparation for birth. The mother may experience increasing discomfort, including back pain, difficulty sleeping, shortness of breath, and Braxton Hicks contractions. The baby's position becomes important as delivery approaches.

Each trimester brings distinct developmental milestones for the baby and physiological changes for the mother. Regular prenatal appointments help track progress through these trimesters and address any concerns that arise.

Key Developmental Milestones by Week

Here are some of the significant developmental milestones that occur throughout pregnancy:

  • Weeks 1-4: Fertilization occurs, and the fertilized egg (zygote) travels to the uterus and implants into the uterine lining. The placenta begins to form.
  • Weeks 5-8: The embryo's heart begins to beat. Brain, spinal cord, and other neural tissue begin to form. Tiny limb buds appear that will develop into arms and legs.
  • Weeks 9-12: External genitalia begin to form. The face has developed features. The embryo is now called a fetus, and all essential organs have begun to form.
  • Weeks 13-16: The fetus begins to move, though the mother usually can't feel it yet. Sex determination is possible via ultrasound. The fetus can make facial expressions.
  • Weeks 17-20: The mother typically begins to feel movement ("quickening"). Growth accelerates, and the baby develops a protective coating called vernix.
  • Weeks 21-24: The fetus has a chance of survival outside the womb (viability), though would require intensive medical care. The lungs begin to develop surfactant, necessary for breathing.
  • Weeks 25-28: The brain develops rapidly. Eyes begin to open. The baby can hear sounds from the outside world.
  • Weeks 29-32: Bones are fully developed but still soft. The baby gains significant weight and can regulate its own body temperature.
  • Weeks 33-36: The baby's lungs are nearly mature. Most babies will turn to a head-down position in preparation for birth.
  • Weeks 37-40: The baby is considered full-term and typically ready for birth. Final organ maturation occurs, and the baby continues to gain weight.

Remember that development is a continuous process, and these milestones represent general timeframes rather than exact points.

Understanding Ultrasound Dating

Ultrasound dating is considered the most accurate method for determining gestational age, particularly when performed in the first trimester. Here's how it works:

  • First Trimester (up to 13 weeks): Crown-rump length (CRL) measurement is used, with accuracy within 5-7 days.
  • Second Trimester (14-26 weeks): Multiple measurements are taken, including head circumference, abdominal circumference, and femur length. Accuracy is within 7-10 days.
  • Third Trimester (after 27 weeks): The same measurements are used, but accuracy decreases to within 2-3 weeks due to normal variations in fetal growth rates.

When there's a discrepancy between LMP dating and ultrasound dating, healthcare providers follow these general guidelines:

  • If the difference is more than 5 days in the first trimester, the ultrasound date is typically used.
  • If the difference is more than 7-10 days in the second trimester, the ultrasound date is typically used.
  • If the difference is more than 14-21 days in the third trimester, the care provider may consider using the ultrasound date or may recommend additional assessment.

While ultrasound dating is accurate, it's important to remember that it provides an estimate rather than an exact determination of gestational age. The earlier in pregnancy the ultrasound is performed, the more accurate the dating will be.

Picture of Dr. Evelyn Carter

Dr. Evelyn Carter

Author | Chief Calculations Architect & Multi-Disciplinary Analyst

Table of Contents

Gestational Age Calculator: Track Your Pregnancy Journey with Precision

Our comprehensive gestational age calculator helps you determine exactly how far along your pregnancy is, your estimated due date, and important developmental milestones. Whether you’re calculating based on your last menstrual period, known conception date, or ultrasound measurements, this tool provides accurate information to guide you through your pregnancy journey.

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Why Gestational Age Matters in Pregnancy

Gestational age is the most important metric used by healthcare providers to track pregnancy progress, assess fetal development, and plan appropriate prenatal care. Understanding your exact gestational age helps with:

Key Benefits of Knowing Your Gestational Age

  • Accurate due date prediction – Helps you plan for your baby’s arrival with greater certainty
  • Proper timing of prenatal tests – Ensures screenings and diagnostic tests occur at optimal times
  • Monitoring fetal growth – Allows healthcare providers to compare your baby’s size against expected norms
  • Planning for appropriate interventions – Guides decisions if complications arise or if delivery timing becomes critical
  • Understanding developmental stages – Helps you follow your baby’s developmental journey week by week

While the traditional 40-week pregnancy timeline is well-known, understanding the nuances of how gestational age is calculated and what it means for your particular pregnancy can provide valuable insights. This calculator combines medical standards with flexibility to accommodate different starting points for calculation.

Understanding the Science of Pregnancy Dating

Pregnancy dating involves several established methods, each with specific applications and accuracy levels. Our calculator incorporates these medical standards to provide comprehensive results:

Naegele’s Rule: The Traditional Method

This classic calculation method has been used for over 200 years and remains standard practice:

  • Adds 280 days (40 weeks) to the first day of the last menstrual period
  • Assumes a 28-day menstrual cycle with ovulation on day 14
  • Adjusts for longer or shorter cycles by adding or subtracting days

While simple, this method has proven remarkably reliable for most pregnancies and serves as the foundation for pregnancy dating worldwide.

Conception-Based Calculation

When conception date is known (such as with assisted reproduction):

  • Adds 266 days (38 weeks) to the conception date
  • Provides high accuracy when conception timing is documented
  • Especially useful for IVF pregnancies or when ovulation tracking has been precise

This method acknowledges that actual embryonic development begins at conception, approximately two weeks after the LMP in a typical cycle.

Ultrasound Dating

Modern ultrasound measurements provide objective data for pregnancy dating:

  • First trimester dating: Most accurate (±5-7 days) using crown-rump length
  • Second trimester: Good accuracy (±10-14 days) using multiple fetal measurements
  • Third trimester: Less precise (±21-30 days) due to normal variations in growth rates

Healthcare providers typically use ultrasound dating to confirm or adjust LMP-based calculations, especially when there’s a significant discrepancy.

Combination Methods

Best clinical practice often involves integrating multiple data points:

  • Early pregnancy: LMP date with cycle length adjustment
  • After ultrasound: Modified dating if significant discrepancy exists
  • Later pregnancy: Sequential growth measurements to ensure appropriate development

Our calculator allows you to enter ultrasound data to refine your estimated gestational age and due date for the most accurate assessment.

Understanding Your Gestational Age Results

Our calculator provides comprehensive information about your pregnancy timeline. Here’s how to interpret the key results:

Current Gestational Age

What it shows: Weeks and days since the beginning of pregnancy (from LMP or adjusted from conception date)

How it’s used: This is your “pregnancy age” – the standard measure healthcare providers use to track your pregnancy

What to know: Remember that the first two weeks of gestational age occur before conception in normal dating conventions

Estimated Due Date (EDD)

What it shows: The calculated date when your pregnancy will reach 40 weeks

How it’s used: Helps you and your healthcare team plan for delivery and maternal leave

What to know: Only about 4% of babies are born exactly on their due date – most arrive within two weeks before or after

Trimester Information

What it shows: The date ranges for your three trimesters

How it’s used: Helps you understand the major phases of pregnancy and their typical characteristics

What to know: Each trimester brings distinct developmental milestones and potential maternal symptoms

Key Pregnancy Milestones

What it shows: Important dates such as viability (24 weeks) and full term (37 weeks)

How it’s used: Provides a framework for understanding critical points in fetal development

What to know: These milestones help guide medical decision-making if complications arise

While these calculations provide valuable guidance, remember that every pregnancy is unique. Regular prenatal care is essential for monitoring your specific pregnancy progression.

Special Considerations for Different Pregnancy Situations

Our calculator accommodates various pregnancy scenarios to provide the most relevant information for your situation:

Irregular Menstrual Cycles

  • Standard calculations assume a 28-day cycle with ovulation on day 14
  • If your cycles are consistently longer or shorter, our calculator adjusts by using your specific cycle length
  • For very irregular cycles, an early ultrasound provides the most accurate dating

If your cycles vary significantly from month to month, use the average length of your last three cycles before pregnancy for the most accurate result.

IVF and Assisted Reproduction

  • Use the “Conception Date” option with the embryo transfer date
  • For frozen embryo transfers, the embryo age should be added to calculate the true gestational age
  • Due dates from IVF are typically very accurate since conception timing is precisely documented

Special note: For day-5 blastocyst transfers, the embryo is already 5 days old at transfer, which should be accounted for in calculations.

Multiple Pregnancies

  • Gestational age calculation methods are the same as for singleton pregnancies
  • Due dates for twins are typically earlier (around 37 weeks) than singletons
  • Growth patterns may differ, so ultrasound measurements become especially important

While our calculator provides standard dating information, consult with your healthcare provider for specific guidance with multiples.

Ultrasound Dating Discrepancies

  • Minor differences (within 5-7 days in first trimester) between LMP and ultrasound dating are normal
  • Larger discrepancies may indicate irregular ovulation or measurement variations
  • Our calculator allows you to incorporate ultrasound information to refine your dates

Generally, if ultrasound dating differs significantly from LMP dating, healthcare providers will use the ultrasound date, especially for first-trimester scans.

Developmental Milestones by Gestational Age

Understanding what’s happening with your baby at each stage can make your pregnancy journey more meaningful. Here are key developmental milestones by gestational age:

First Trimester (Weeks 1-12)

  • Weeks 1-2: Pre-conception phase in standard dating; your body prepares for pregnancy
  • Weeks 3-4: Fertilization and implantation occur; pregnancy hormone production begins
  • Weeks 5-6: The embryonic heart begins beating; neural tube forms
  • Weeks 7-8: Tiny limb buds appear; facial features begin forming
  • Weeks 9-10: External genitalia begin developing; all essential organs are forming
  • Weeks 11-12: Fetal movement begins (though not yet felt); sex may be visible on ultrasound

During this critical period, your baby grows from a microscopic cell to about 2.5 inches long and weighs approximately half an ounce.

Second Trimester (Weeks 13-26)

  • Weeks 13-16: Fingerprints form; baby begins making facial expressions
  • Weeks 17-20: You’ll likely feel movement (“quickening”); vernix and lanugo develop
  • Weeks 21-22: Eyes complete formation; sleep cycles develop
  • Weeks 23-24: Viability threshold; lung development accelerates with surfactant production
  • Weeks 25-26: Brain growth surges; baby responds to sounds from outside

The second trimester sees rapid growth, with your baby reaching about 14 inches and 1.5-2 pounds by the end.

Third Trimester (Weeks 27-40+)

  • Weeks 27-30: Brain development accelerates; baby can open and close eyes
  • Weeks 31-34: Rapid weight gain; immune system development
  • Weeks 35-36: Lungs nearly mature; baby usually moves to head-down position
  • Weeks 37-38: Considered “early term”; most organ systems are mature
  • Weeks 39-40: “Full term”; average birth weight of 7-8 pounds
  • Weeks 41-42: “Late term”; placental function may begin to decline

During the final trimester, your baby gains significant weight, with refinement of all body systems in preparation for life outside the womb.

Using Gestational Age to Navigate Prenatal Care

Accurate gestational age calculation helps you navigate the recommended schedule for prenatal care, tests, and preparations. Here’s how gestational age guides your pregnancy journey:

First Trimester Prenatal Care (Weeks 1-12)

  • Weeks 4-8: Confirm pregnancy and establish prenatal care
  • Weeks 8-10: First prenatal appointment with medical history and baseline assessments
  • Weeks 10-13: Optional genetic screening tests (NIPT, NT scan)
  • Throughout: Prenatal vitamins, especially folic acid; dietary and lifestyle adjustments

Early care focuses on confirming pregnancy health, establishing accurate dating, and identifying any risk factors that may affect your pregnancy.

Second Trimester Care (Weeks 13-26)

  • Weeks 15-20: Maternal serum screening tests
  • Weeks 18-22: Detailed anatomy ultrasound to check baby’s development
  • Weeks 24-28: Glucose screening test for gestational diabetes
  • Throughout: Monthly prenatal visits to check growth, heart tones, and maternal health

Middle pregnancy focuses on monitoring normal development and detecting any potential complications that could affect later pregnancy.

Third Trimester Care (Weeks 27-40+)

  • Weeks 28-36: Biweekly prenatal visits
  • Week 36 onward: Weekly visits until delivery
  • Weeks 35-37: Group B streptococcus (GBS) screening
  • Throughout: Monitoring for preeclampsia, fetal position, and cervical changes

Late pregnancy care focuses on monitoring for labor signs, ensuring optimal conditions for delivery, and preparing for birth.

Planning for Delivery Based on Gestational Age

  • Before 37 weeks: Considered preterm; medical interventions may be needed to delay birth
  • 37-38 weeks: Early term; delivery may be considered for certain medical indications
  • 39-40 weeks: Full term; ideal timeframe for spontaneous delivery
  • 41 weeks: Late term; increased monitoring often recommended
  • Beyond 42 weeks: Post-term; induction generally recommended due to increased risks

Your healthcare provider will use gestational age as a key factor in making recommendations about timing and method of delivery.

Common Questions About Gestational Age

Why is pregnancy calculated from the last period and not conception?

Pregnancy is traditionally calculated from the last menstrual period (LMP) rather than conception for several practical reasons. First, most women can recall the date of their last period more accurately than their conception date. Second, ovulation and conception timing vary greatly between women and even between cycles for the same woman. Third, this convention established a consistent 40-week timeline that healthcare providers worldwide understand. It’s worth noting that this means the first two weeks of “pregnancy” by this calculation occur before conception has actually taken place. While this may seem counterintuitive, this standard dating system has proven reliable for clinical practice. For those who prefer conception-based dating, simply subtract two weeks from the standard gestational age (assuming a typical 28-day cycle with ovulation on day 14).

How accurate are due dates calculated from the last menstrual period?

Due dates calculated from the last menstrual period (LMP) have a natural margin of error. Only about 4% of babies are born exactly on their calculated due date, with about 70% arriving within 10 days of it (either before or after). Several factors affect this accuracy: variations in cycle length, irregular periods, inconsistent ovulation timing, and differences in natural pregnancy duration between women. For women with very regular 28-day cycles who are certain of their LMP date, the calculation may be accurate within 5-7 days. For those with irregular cycles, the margin of error increases. This is why healthcare providers often use early ultrasounds to confirm or adjust LMP-based due dates. First-trimester ultrasounds can provide dating accuracy within 5-7 days, while second-trimester scans have a wider margin of 10-14 days. Remember that due dates are best viewed as the middle of a “due month” rather than a specific day.

What happens if there’s a discrepancy between LMP dating and ultrasound dating?

Discrepancies between LMP-based dating and ultrasound dating are common, occurring in roughly 15-20% of pregnancies. Healthcare providers follow established guidelines when reconciling these differences. In the first trimester, if the ultrasound date differs by more than 5-7 days from the LMP date, the ultrasound date is typically adopted. In the second trimester (up to 20 weeks), differences greater than 7-10 days usually lead to using the ultrasound date. Beyond 20 weeks, ultrasound dating becomes less reliable for changing established dates due to normal variations in fetal growth rates. These discrepancies don’t necessarily indicate a problem—they often result from irregular ovulation timing, inaccurate recall of the LMP, or normal biological variations. When a significant adjustment is made, your healthcare provider will explain the rationale and update your records with a new estimated due date that will guide future care decisions.

How does gestational age differ from fetal age?

Gestational age and fetal age refer to different starting points for pregnancy timing, which creates a consistent two-week difference between them. Gestational age, the standard clinical measurement, counts from the first day of the last menstrual period (LMP)—approximately two weeks before conception occurs in a typical cycle. Fetal age (also called embryonic age) counts from the actual conception or fertilization date, marking the true beginning of embryonic development. For example, a pregnancy described as “10 weeks gestational age” would be “8 weeks fetal age.” This distinction explains why pregnancy is said to last 40 weeks (gestational age) rather than 38 weeks (fetal age). The gestational age system is predominant in clinical settings worldwide, while embryologists and fertility specialists may use fetal age when discussing early development. Our calculator accommodates both systems by allowing calculation from either LMP or conception date, though results are typically expressed in gestational age to align with standard medical practice.

How do I calculate gestational age with IVF or fertility treatments?

Calculating gestational age with IVF or fertility treatments requires specific adjustments to account for the known conception timeline. For fresh embryo transfers, use the retrieval/fertilization date as the conception date in our calculator. For frozen embryo transfers, use the transfer date but add the embryo’s age at freezing (typically 3 or 5 days) to determine the true conception equivalent. While the calculator will add the standard two pre-conception weeks to align with conventional gestational age, your reproductive endocrinologist may use different dating conventions during your fertility treatment. After graduating to regular prenatal care, providers typically convert to standard gestational age dating. IVF pregnancies offer the advantage of precisely known conception timing, which eliminates much of the dating uncertainty present in spontaneous conceptions. If you undergo an early ultrasound that shows a significant discrepancy from your IVF-based dates, discuss this with your healthcare provider, as IVF dating is generally considered highly accurate.

Related Pregnancy and Health Calculators

Continue your pregnancy planning with these complementary calculators:

Scientific Evidence Behind Gestational Age Calculation

The methods used in our gestational age calculator are based on established obstetrical research and clinical guidelines:

  • Naegele’s rule, dating from 1812, has been validated by modern studies as a reasonable approximation for pregnancies with typical 28-day cycles, with expected biological variations of ±2 weeks.
  • Research published in the American Journal of Obstetrics and Gynecology confirms that first-trimester ultrasound measurements, particularly crown-rump length, provide the most accurate pregnancy dating (±5-7 days).
  • The American College of Obstetricians and Gynecologists (ACOG) guidelines recommend redating pregnancies when first-trimester ultrasound measurements differ by more than 5-7 days from LMP dating.
  • Studies published in the Journal of Ultrasound in Medicine demonstrate that early dating ultrasounds improve outcome predictions and reduce the incidence of post-term pregnancy diagnoses.
  • Research from the National Institute of Child Health and Human Development Fetal Growth Studies has established standardized fetal growth curves that inform the growth estimations in our calculator.

This evidence-based approach ensures our calculator provides reliable gestational age estimates that align with current obstetrical standards of care.

Health Disclaimer

The Gestational Age Calculator and accompanying information are provided for educational purposes only. This tool is not intended to replace professional medical advice, diagnosis, or treatment.

While gestational age calculation is an important part of pregnancy monitoring, it should be considered alongside regular prenatal care with qualified healthcare providers. Individual variations in menstrual cycles, conception timing, and fetal development may affect the accuracy of these calculations.

Always consult with qualified healthcare professionals for personalized advice regarding your pregnancy. Only your healthcare provider can make specific recommendations based on your complete medical history and current condition.

Last Updated: March 29, 2025 | Next Review: March 29, 2026