Gestational Age Calculator

Gestational Age Calculator – Free Pregnancy Week Calculator by LMP, Ultrasound & Conception | FreeAgeCalculatorPro
LMP · Conception · Ultrasound · Cycle Adjustment · Naegele's Rule

Free Gestational Age Calculator — How Many Weeks Pregnant?

Instantly calculate your gestational age in weeks and days, your current trimester, estimated due date, and days remaining — using LMP (Naegele's Rule), conception date, or ultrasound scan date. Includes cycle length adjustment for irregular periods.

  • ✓
    Three methods: LMP, conception date, or ultrasound scan
  • ✓
    Cycle length adjustment for cycles other than 28 days
  • ✓
    Shows trimester, EDD, days remaining, and fetal size hint
  • ✓
    Free worldwide — no signup, no registration
🗓️ LMP Method 🔬 Conception Date 📡 Ultrasound Scan

🤰 Gestational Age Calculator

Choose a method and enter your dates for instant results

The first day of your most recent period
Default: 28 days
Default: today
Known ovulation date, IUI, or IVF retrieval date
Default: today
Date the ultrasound was performed
Default: today
—
—
Total Weeks
—
Total Days
—
Est. Due Date
—
Days Remaining
⚕️ For informational use only. Consult your healthcare provider for clinical dating.
✓ 3 Methods ✓ Cycle Adjusted ✓ EDD Included ✓ 100% Free
LMP — Naegele's Rule
Conception / Ovulation Date
Ultrasound Scan Dating
Cycle Length Adjustment
100% Free · No Signup
How It Works

Calculate Your Gestational Age
in Four Simple Steps

Choose the method that matches what you know — LMP, conception date, or ultrasound — and get your gestational age instantly.

STEP 01
📋

Choose Your Method

Select LMP (Naegele's Rule), conception date, or ultrasound scan date depending on which dates you have available and which is most accurate for you.

STEP 02
📅

Enter Your Dates

For LMP: enter the first day of your last period and cycle length. For conception: enter ovulation/IVF date. For ultrasound: enter scan date and gestational age from the report.

STEP 03
⚡

Click Calculate

The calculator applies Naegele's Rule (LMP method) or the standard clinical offset formula to instantly compute your gestational age, trimester, and estimated due date.

STEP 04
📊

View Full Results

See gestational age in weeks and days, your current trimester, EDD, days remaining until delivery, and a fetal size hint for your current week.

Trimester Guide

The Three Trimesters of Pregnancy —
Week-by-Week Overview

A full-term pregnancy spans 40 weeks divided into three distinct trimesters, each with key developmental milestones and recommended prenatal care.

🔵 First Trimester
Weeks 1 – 13

The most critical period for organ development. Morning sickness, fatigue, and breast tenderness are common. The embryo develops into a fetus by week 10.

  • Week 4–5: Implantation; heart begins beating
  • Week 6–8: Brain, spinal cord, and organs forming
  • Week 9–10: Fingers, toes, facial features visible
  • Week 11–13: Nuchal translucency scan; risk of miscarriage drops significantly
🟣 Second Trimester
Weeks 14 – 27

Often called the "golden trimester" — most women feel better as nausea subsides. The baby grows rapidly and movements become noticeable around weeks 18–20.

  • Week 14–16: Baby moves limbs; gender may be detectable
  • Week 18–20: Anatomy scan (anomaly scan) recommended
  • Week 22–24: Baby can hear sounds; lungs developing
  • Week 24: Viability threshold — survival possible with NICU support
🟡 Third Trimester
Weeks 28 – 40+

Rapid weight gain for baby; lungs mature; brain development accelerates. Braxton Hicks contractions may begin. Full term is reached at 39–40 weeks.

  • Week 28–32: Baby's eyes open; sleep cycles begin
  • Week 34–36: Baby usually moves into head-down position
  • Week 37: Early term — lungs and brain nearly complete
  • Week 39–40: Full term — optimal time for safe delivery
Dating Methods

Three Methods for Calculating
Gestational Age — Compared

Different situations call for different dating methods. Understanding the accuracy and limitations of each helps you choose the best approach for your pregnancy.

METHOD 01
🗓️

LMP — Last Menstrual Period

The most widely used method. Gestational age is counted from the first day of your last period using Naegele's Rule: LMP + 280 days = EDD. Best when you have regular cycles and know your LMP precisely.

Limitation: Assumes a 28-day cycle with ovulation on day 14. Less accurate for irregular cycles.

Accuracy: ±7–14 days (regular cycles) / Less for irregular
METHOD 02
🔬

Conception / Ovulation Date

If you know your ovulation date through temperature charting, OPK testing, or medically assisted conception (IUI, IVF), clinical gestational age is set at conception date + 14 days.

Best for: IVF patients, those using ovulation tracking, or assisted reproduction procedures.

Accuracy: ±3–7 days with confirmed ovulation
METHOD 03
📡

Ultrasound (Dating Scan)

First-trimester ultrasound using Crown-Rump Length (CRL) is the gold standard for pregnancy dating. The scan date and gestational age from the report are used to calculate current gestational age.

Best for: Cases where LMP is unknown, irregular, or when LMP and ultrasound disagree by >7 days.

Accuracy: ±3–5 days (1st trimester CRL)
Fetal Development

Week-by-Week Fetal Size &
Development Guide

Approximate fetal size and key developmental events at key gestational weeks. All sizes are approximate — every pregnancy is unique.

Gestational Week Trimester Fetal Size (Approx.) Size Comparison Key Development
4 Weeks 1st ~2 mm Poppy seed Implantation; embryonic disc forms
6 Weeks 1st ~6 mm Sweet pea Heart beating; brain and spinal cord forming
8 Weeks 1st ~16 mm Raspberry All major organs forming; limb buds present
10 Weeks 1st ~31 mm Strawberry Fetal stage begins; fingers and toes forming
12 Weeks 1st ~55 mm Lime Nuchal translucency scan window; kidneys producing urine
16 Weeks 2nd ~12 cm Avocado Facial muscles developing; gender identifiable
20 Weeks 2nd ~16 cm Banana Anatomy scan week; baby can hear sounds
24 Weeks 2nd ~21 cm Ear of corn Viability threshold; lungs producing surfactant
28 Weeks 3rd ~25 cm Eggplant Eyes can open; brain developing rapidly
32 Weeks 3rd ~29 cm Large squash Fingernails formed; sleep cycles established
36 Weeks 3rd ~34 cm Head of lettuce Late preterm; baby typically in head-down position
40 Weeks 3rd ~50 cm Watermelon Full term; ready for birth; ~3.4 kg average weight
Prenatal Testing

Key Prenatal Tests by
Gestational Age Week

Your gestational age determines which prenatal tests and appointments are recommended. Knowing your exact gestational age helps you prepare for each milestone on time.

🩺
Weeks 8–10

First Prenatal Visit & Booking Appointment

Blood tests (blood type, full blood count, rubella immunity, STI screening), urine culture, blood pressure baseline, and booking ultrasound if not already done. Medical history reviewed.

📡
Weeks 11–13

Nuchal Translucency (NT) Scan

Ultrasound measures fluid at the back of the baby's neck (nuchal fold). Combined with blood tests (PAPP-A and free β-hCG), this gives a risk assessment for chromosomal conditions including Down syndrome. Most accurate window for dating by CRL.

🧬
Weeks 10–13

NIPT & CVS (If Indicated)

Non-invasive prenatal testing (NIPT) screens cell-free fetal DNA in maternal blood for chromosomal conditions. Chorionic villus sampling (CVS) is an invasive diagnostic test offered when NIPT or NT results indicate higher risk.

🔬
Weeks 15–20

Maternal Serum Screening (Quad Screen)

Blood test measuring AFP, hCG, estriol, and inhibin-A to assess risk of Down syndrome, trisomy 18, and neural tube defects. Offered when NIPT was not done or as additional information alongside first-trimester screening.

🩻
Weeks 18–22

Anatomy Scan (Mid-Pregnancy Ultrasound)

Detailed ultrasound examination of fetal anatomy — brain, spine, heart, kidneys, limbs, and placental position. This is the most comprehensive structural assessment of the pregnancy. Gender determination is possible at this scan.

🩸
Weeks 24–28

Glucose Challenge Test (GCT) / OGTT

Screening for gestational diabetes. The one-hour glucose challenge test (GCT) is administered first; if abnormal, the three-hour oral glucose tolerance test (OGTT) is performed. Rhesus negative mothers receive their first anti-D injection around week 28.

📋
Weeks 35–37

Group B Streptococcus (GBS) Swab

Vaginal and rectal swab to screen for Group B Strep bacteria, which can be passed to the baby during birth. If positive, IV antibiotics are given during labor. Growth scan may be offered at this stage if there are concerns about fetal size.

🏥
Weeks 38–41

Final Appointments & Post-Dates Planning

Weekly midwife or OB appointments to monitor fetal movement, cervical status, and blood pressure. If pregnancy continues past 41 weeks, induction of labor is typically discussed. Post-term is defined as beyond 42 weeks gestation.

Naegele's Rule

How Naegele's Rule Calculates
Gestational Age & Due Date

Naegele's Rule is the standard obstetric formula for estimating the due date and gestational age from the last menstrual period. Named after German obstetrician Franz Karl Naegele (1778–1851), it has been the clinical standard for over 200 years and remains the foundation of all LMP-based pregnancy dating.

The rule works by adding 280 days (40 weeks) to the first day of the LMP. This corresponds to adding 1 year, subtracting 3 months, and adding 7 days. The underlying assumption is a regular 28-day menstrual cycle with ovulation occurring on day 14. The first two weeks of "pregnancy" as counted by gestational age are actually the weeks before conception — the pre-ovulatory phase of the cycle.

When a woman's cycle length differs from 28 days, the EDD is adjusted by the number of days difference. A 35-day cycle means ovulation occurs approximately 7 days later than assumed, shifting the EDD 7 days forward. Our calculator applies this adjustment automatically.

Despite its age, Naegele's Rule remains remarkably accurate for women with regular cycles. For irregular cycles, unknown LMP dates, or assisted reproduction pregnancies, first-trimester ultrasound is the preferred dating method and takes precedence when the two methods disagree by more than 5–7 days.

📐 Naegele's Rule — Step by Step

// Standard Naegele's Rule formula
EDD = LMP + 280 days

// Equivalent manual calculation:
EDD = LMP + 1 year − 3 months + 7 days

// Cycle length adjustment:
EDD_adjusted = EDD + (cycleLength − 28)

// Current gestational age:
GA_days = today − LMP − cycle_offset
GA_weeks = GA_days ÷ 7
1
Example: LMP = January 15, 2026 (28-day cycle)Add 280 days → EDD = October 22, 2026
2
If cycle is 35 days (7 days longer)EDD shifts forward by 7 days → October 29, 2026
3
Today is June 2, 2026 (138 days after LMP)GA = 138 days ÷ 7 = 19 weeks 5 days — Second Trimester
4
Days remaining = EDD − todayOctober 22 − June 2 = 142 days remaining until EDD
Related Tools

Explore More Free
Age Calculators

FreeAgeCalculatorPro offers a complete suite of free age and date tools for every personal and professional need.

🗓️
Age CalculatorExact age from any date of birth
→
🍼
Corrected Age CalculatorPost-birth age for premature babies
→
📅
Adjusted Age CalculatorCorrected age from DOB & due date
→
📋
ASQ Age CalculatorAges & Stages Questionnaire intervals
→
📆
Chronological Age CalculatorPrecise age from birth to any date
→
⚖️
Age Difference CalculatorCompare age between two dates
→
🧬
Biological Age CalculatorEstimate your body's real age
→
🎂
Birthday Countdown CalculatorDays until your next birthday
→
In-Depth Guide

Complete Guide to
Gestational Age Calculation

What Is Gestational Age?

Gestational age is the standard clinical measure of how far along a pregnancy has progressed. It is counted in weeks and days from the first day of the mother's last menstrual period (LMP) — not from the date of conception. This dating convention means that a woman is technically considered "2 weeks pregnant" at the time of ovulation, because the first two weeks of gestational age cover the pre-ovulatory phase of the menstrual cycle.

A full-term pregnancy is considered to be 40 weeks (280 days) of gestational age. In practice, only about 5% of babies are born on their exact due date — most births occur within a window of two weeks before or after the EDD.

Gestational Age vs Fetal Age — Key Difference

Gestational age and fetal age (also called developmental age or embryonic age) are not the same. Fetal age counts from the date of fertilization — which occurs approximately 14 days after the LMP in a 28-day cycle. This means gestational age is approximately 2 weeks longer than fetal age. A pregnancy that is 12 weeks gestational age is approximately 10 weeks of fetal age.

All clinical practice uses gestational age, not fetal age, because the LMP is a known date while the exact date of fertilization is rarely known. All prenatal tests, developmental milestones, and growth charts are calibrated to gestational age.

When LMP Dating and Ultrasound Dating Disagree

It is common for LMP-based gestational age and ultrasound-based gestational age to differ by a few days — especially when cycles are irregular or the LMP date is uncertain. The American College of Obstetricians and Gynecologists (ACOG) recommends that when first-trimester ultrasound (Crown-Rump Length measurement) disagrees with LMP dating by more than 5–7 days, the ultrasound dating should be used to establish the EDD. This is because early ultrasound is more accurate than LMP recall for most women.

In the second trimester, the threshold for changing the EDD based on ultrasound increases to 10–14 days. In the third trimester, ultrasound dating is considerably less accurate and is generally not used to revise the EDD.

How Gestational Age Affects Prenatal Care Decisions

Gestational age is the most important variable in prenatal care. It determines when specific blood tests and ultrasounds must be scheduled (many have narrow optimal windows), when medications are safe or contraindicated, what the risks of complications are at any given point, whether a birth is considered preterm, term, or post-term, and NICU management decisions if the baby is born early.

This is why accurate gestational age dating at the earliest possible stage of pregnancy is a clinical priority — the accuracy of all subsequent care decisions depends on it.

⚠️ Medical Disclaimer: This gestational age calculator is provided for informational and educational purposes only. It does not constitute medical advice. Gestational age dating for clinical purposes should always be established by a qualified healthcare provider using appropriate clinical methods. Never use this calculator as a substitute for professional prenatal care.

FAQ

Frequently Asked Questions

Clear answers to the most common questions about gestational age, pregnancy dating, and our free calculator.

Gestational age measures how far along a pregnancy is, counted in weeks from the first day of the mother's last menstrual period (LMP). It is approximately 2 weeks longer than the actual fetal age (time since fertilization), because the LMP is 2 weeks before ovulation in a standard cycle. After birth, gestational age at birth is used to classify whether a baby is premature, full-term, or post-term.
Naegele's Rule is the standard formula for estimating the due date: EDD = LMP + 280 days (40 weeks). It is named after German obstetrician Franz Karl Naegele and assumes a regular 28-day cycle with ovulation on day 14. For women with regular 28-day cycles and a known LMP, it is accurate to within about 7–14 days. For irregular cycles, it is less precise. Our calculator adjusts the EDD for cycles that differ from 28 days.
Naegele's Rule assumes ovulation on day 14 of a 28-day cycle. If your cycle is longer — say 35 days — ovulation occurs around day 21, which is 7 days later than assumed. This means conception occurred 7 days later, and your due date shifts 7 days forward. Our calculator adjusts the EDD by subtracting 28 from your cycle length and adding the difference in days to the standard EDD calculation.
First-trimester ultrasound using Crown-Rump Length (CRL) is the most accurate method, with a margin of error of only ±3–5 days. It is more accurate than LMP-based dating because it does not rely on cycle length assumptions or LMP recall accuracy. If your first-trimester ultrasound and LMP dates disagree by more than 5–7 days, most clinicians will use the ultrasound date to establish the official EDD.
The first trimester spans weeks 1–13 (0 to 13 weeks 6 days of gestational age). The second trimester spans weeks 14–27 (14 to 27 weeks 6 days). The third trimester begins at week 28 and continues until delivery. Full-term pregnancy is defined as 39–40 weeks; early term is 37–38 weeks; late preterm is 34–36 weeks; preterm is before 34 weeks.
For IVF pregnancies, gestational age is calculated from the egg retrieval date or embryo transfer date. For fresh transfers, the equivalent LMP is the egg retrieval date minus 14 days. For frozen embryo transfers (FET), the equivalent LMP is 2 weeks before the transfer date for a day-2 or day-3 embryo, or 2 weeks and 2 days before for a day-5 blastocyst transfer. Use the conception/ovulation date method in our calculator with the equivalent date, or ask your fertility clinic for the confirmed gestational age at the time of the transfer.
Babies born at 37–38 weeks are considered "early term" — they are not premature, but they are slightly less mature than full-term babies and may have a higher risk of feeding difficulties, jaundice, and temperature regulation challenges. Babies born at 39–40 weeks are "full term" — this is the optimal gestational age for birth. Babies born at 41 weeks are "late term" and those at 42 weeks or beyond are "post-term." Full-term (39–40 weeks) is the safest gestational age for delivery.
Yes — completely free, with no account, no registration, and no hidden fees. FreeAgeCalculatorPro is available worldwide without restriction, and will remain free permanently.

Calculate Your Gestational Age Now

Enter your LMP, conception date, or ultrasound data and get your gestational age, trimester, and due date instantly. Free for everyone, forever.

⚡ Calculate Gestational Age Now Corrected Age Calculator