How Many Weeks Pregnant: Calculate Your Pregnancy Week

You just found out you’re pregnant. Congratulations!

Now someone asks: “How far along are you?” And you’re not sure how to answer.

Is it the number of weeks since you had sex? Since your last period? Since you took the pregnancy test? Is there a difference between weeks of pregnancy and weeks since conception?

Yes, there’s a big difference, and it’s confusing.

The good news: Calculating how many weeks pregnant you are is actually pretty simple once you understand the method doctors use. And knowing exactly how far along you are matters for your health, your doctor’s appointments, and understanding what’s happening with your baby right now.

This guide will clear up the confusion and help you figure out exactly how far along you are—plus what you should expect at each stage of your pregnancy.

The Two Ways to Count Pregnancy Weeks

This is where the confusion starts. There are two different ways to count pregnancy weeks, and they’re measured from different points.

Gestational Age (What Your Doctor Uses)

Definition: The age of your pregnancy calculated from the first day of your last menstrual period (LMP)

Why this method:

• Doctors use this to track pregnancy development
• This is the standard medical measurement
• This is what your due date is based on
• Ultrasounds confirm (or adjust) this measurement

Starting point: First day of your last period (before conception happened)

Example: If your last period started on January 1st, and today is February 12th, you are approximately 6 weeks pregnant (gestational age).

Fertilization Age (Conception Age)

Definition: The age of your pregnancy calculated from the date you conceived

Why people use this:

• Feels more “accurate” (counting from actual conception)
• More intuitive for some people
• Conception-based

Starting point: The actual day you had intercourse and conceived

Important note: Fertilization age is always about 2 weeks younger than gestational age

Example: If you conceived on January 15th, and today is February 12th, you are approximately 4 weeks old (fertilization age)

Why Your Doctor Uses Gestational Age

This is important: Doctors measure pregnancy in gestational age, not fertilization age.

Why Gestational Age Matters

1. Due date calculation – Based on gestational age (280 days from LMP)
2. Screening tests – Timed based on gestational weeks
3. Ultrasound measurements – Baby size confirms gestational age
4. When labor is “overdue” – Counted from gestational age
5. Baby’s development timeline – Described in gestational weeks

Why Not Fertilization Age?

• You don’t always know your exact conception date
• Many women ovulate on different days
• Sperm can survive several days before fertilization
• It’s harder to track accurately

How to Calculate Your Gestational Age (Due Date Method)

This is the simplest way to figure out how far along you are.

Step 1: Find Your Last Menstrual Period Date

The first day of your last period is your starting point.

Don’t remember exactly? Look at:

• Calendar or phone (did you track it?)
• Period tracking app (Clue, Flo, Period Tracker)
• When you last used tampons/pads
• Doctor records from past exams

If you’re not sure: Tell your doctor. They can estimate from ultrasound.

Step 2: Count Forward from That Date

Quick calculation:

• Add 280 days to the first day of your last period = Your due date
• Divide remaining days by 7 = Your current week

Or use the simpler method:

Add 9 months and 7 days to your LMP date

Example:

• Last period: January 1, 2026
• Add 9 months: October 1, 2026
• Add 7 days: October 8, 2026 ← Your due date
• If today is February 12: You are about 6 weeks pregnant

Step 3: Use a Pregnancy Calculator

The easiest way: Use our Pregnancy Calculator tool

Just enter: First day of last menstrual period It calculates:

• How many weeks pregnant you are
• Your approximate due date
• Which trimester you’re in
• Baby’s approximate size

Pregnancy by the Weeks: What to Expect

Understanding which week you’re at helps you know what’s happening with your baby and your body.

First Trimester (Weeks 1-13)

Weeks 1-2: Before You’re Actually Pregnant

• Baby stage: Egg hasn’t been fertilized yet
• What’s happening: Your body is preparing to ovulate
• Your symptoms: Normal menstrual cycle
• What to do: Track ovulation if trying to conceive
• Why this matters: This is counted in your “pregnancy” even though you’re not technically pregnant yet

Weeks 3-4: Conception & Implantation

• Baby stage: Sperm meets egg, fertilization occurs
• What’s happening: Fertilized egg (zygote) becomes blastocyst, travels to uterus, implants
• Your symptoms: You might feel nothing yet
• Baby size: Smaller than a poppy seed
• What to do: If trying to conceive, wait for symptoms

Weeks 5-6: First Pregnancy Confirmation

• Baby stage: Embryo
• What’s happening: Embryo grows, heart begins beating, neural tube formation
• Your symptoms: Missed period, breast tenderness, fatigue, nausea starting
• Baby size: Pea-sized (about 4mm)
• What to do: Take pregnancy test, schedule first doctor appointment

Weeks 7-8: Major Development

• Baby stage: Embryo with recognizable features
• What’s happening: Brain develops, facial features form, limbs budding
• Your symptoms: Nausea peak, fatigue, food aversions
• Baby size: Blueberry (about 12-16mm)
• What to do: First prenatal appointment (usually around week 8)

Weeks 9-10: Embryo Becomes Fetus

• Baby stage: Now called a fetus (major development complete)
• What’s happening: Fingers and toes visible, brain activity detectable
• Your symptoms: Nausea continues, exhaustion, frequent urination
• Baby size: Olive or grape (about 22-30mm)
• What to do: Dating ultrasound to confirm due date

Weeks 11-12: Rapid Growth

• Baby stage: Fetus rapidly growing
• What’s happening: Nail beds forming, bones hardening, organs functioning
• Your symptoms: Nausea may improve, energy returning
• Baby size: Plum (about 40-50mm, weigh 8-14 grams)
• What to do: End of first trimester, risk of miscarriage drops significantly

Week 13: End of First Trimester

• Baby stage: Fetus with recognizable human features
• What’s happening: All organs present, bone ossification beginning
• Your symptoms: Morning sickness usually improving, energy returning
• Baby size: Peach (about 65-75mm, weigh 23g)
• What to do: Some women announce pregnancy at this milestone

Second Trimester (Weeks 14-27)

Weeks 14-16: The “Honeymoon” Period

• Baby stage: Features more refined, starts moving
• What’s happening: Baby’s movements may become noticeable (fluttering)
• Your symptoms: Energy back, morning sickness gone, appetite returned
• Baby size: Lemon (about 3.5-4.5 inches)
• What to do: Anatomy ultrasound around week 18-20

Weeks 17-20: Anatomy Scan

• Baby stage: Baby can be measured to confirm growth
• What’s happening: Anatomy ultrasound shows baby’s organs, position, sex (if you want to know)
• Your symptoms: Feeling great, baby movements obvious
• Baby size: Cantaloupe or tomato (about 5.5 inches)
• What to do: Most important ultrasound—checks baby’s development

Weeks 21-24: Viability Milestone

• Baby stage: Baby is “viable” (could survive with medical help if born)
• What’s happening: Baby develops sleep-wake cycles, hears sounds
• Your symptoms: Braxton Hicks contractions start (not real labor)
• Baby size: Large mango or papaya (about 7.5 inches)
• What to do: Continue regular prenatal care, watch for complications

Weeks 25-27: Third Trimester Approaches

• Baby stage: Rapid weight gain begins, brain development continues
• What’s happening: Baby positioning may change, more kick strength
• Your symptoms: Back pain, heartburn, leg cramps, difficulty sleeping
• Baby size: Large papaya or cauliflower (about 9 inches, 2 lbs)
• What to do: Monitor for gestational diabetes, prepare for labor

Third Trimester (Weeks 28-42)

Weeks 28-30: Baby Gets Bigger

• Baby stage: Baby position matters (head down preferred)
• What’s happening: Baby continues gaining weight rapidly
• Your symptoms: Swelling, shortness of breath, more Braxton Hicks
• Baby size: Large eggplant or butternut squash (about 10-11 inches, 2.5-3 lbs)
• What to do: More frequent doctor visits, discuss labor preferences

Weeks 31-36: Baby Drops

• Baby stage: Baby settles into birth position (usually head down)
• What’s happening: Baby “drops” into pelvis (lightens upper belly)
• Your symptoms: Pelvic pressure, back pain, frequent bathroom trips
• Baby size: Large zucchini or pumpkin (about 11-13 inches, 3.5-5.5 lbs)
• What to do: Pack hospital bag, discuss birth plan

Weeks 37-39: Full Term

• Baby stage: Baby fully developed, ready for birth
• What’s happening: More contractions (Braxton Hicks and real labor signs)
• Your symptoms: Exhaustion, impatience, labor preparation
• Baby size: Large pumpkin (about 13-15 inches, 5.5-7 lbs)
• What to do: Watch for labor signs, doctor visits 1-2x per week

Weeks 40-42: Due Date & Beyond

• Baby stage: Baby ready to be born any day
• What’s happening: Labor can start anytime
• Your symptoms: Every symptom feels like labor, anxiety about overdue
• Baby size: Watermelon (about 19-22 inches, 7-8.5 lbs)
• What to do: Wait patiently (or NOT patiently—everyone’s impatient!), watch for labor signs

Note: Pregnancy lasting until week 42 is considered “postdates” and doctors will discuss induction options.

Trimester Breakdown

First Trimester (Weeks 1-13)

• Length: 13 weeks
• Major milestone: All baby’s organs form
• Mom changes: Nausea, fatigue, breast tenderness
• Miscarriage risk: Highest in first 12 weeks (then drops)
• Screening options: First trimester screening, NIPT

Second Trimester (Weeks 14-27)

• Length: 14 weeks
• Major milestone: Baby becomes viable
• Mom changes: Feeling great, more energy, showing
• Miscarriage risk: Drops significantly
• Key ultrasound: Anatomy scan (weeks 18-20)

Third Trimester (Weeks 28-42)

• Length: 15 weeks
• Major milestone: Baby’s final development and positioning
• Mom changes: Uncomfortable, swollen, ready to be done
• Miscarriage risk: Very low
• Key milestone: Week 37 = full term

Common Confusion About Pregnancy Weeks

“Why Am I Counted as 4 Weeks Pregnant If I Only Conceived 2 Weeks Ago?”

Answer: Because doctors count pregnancy from your last period, not conception.

• Last period: January 1
• Ovulation & conception: Around January 15 (2 weeks later)
• Today (February 12): You’re 6 weeks “pregnant” but only 4 weeks from conception

This is frustrating but medically standard.

“My Ultrasound Shows I’m Further Along Than I Thought”

Why this happens:

• You might have miscalculated your last period
• You ovulated earlier or later than “typical”
• Baby is measuring ahead (perfectly normal—babies grow at different rates)

What to do: Trust the ultrasound dating, especially in first trimester. Your due date may shift slightly.

“I’m Due on [Date] But the Doctor Says My Due Date is Different”

Why this happens:

• Due dates are estimates, not exact dates
• Ultrasound measurements might show different dates
• Different calculation methods (LMP vs ultrasound)

Important: Only 4-5% of babies are born on their actual due date. Being off by 1-2 weeks is totally normal.

“Everyone Says I’m Not as Far Along as I Think”

Reasons:

• Different measurement methods (gestational vs fertilization age)
• Different starting points (people confuse when pregnancy “started”)
• When ultrasound dating differs from LMP dating

What to do: Ask your doctor what date they’re using as your due date. That’s the official one.

Calculating Baby’s Age After Birth

Once your baby is born, the way you calculate age changes.

Newborn to 1 Year

Age is calculated in weeks and months

• Week 1, Week 2, etc.
• Month 1 (4 weeks), Month 2 (8 weeks), etc.

Example:

• Born: January 1
• Today (January 29): Baby is 4 weeks old

Age 1 Year and Older

Age is calculated in years

• 1 year old
• 2 years old
• Etc.

FAQ: Pregnancy Weeks & Due Dates

Q: When is my actual due date?

A: Due date is 280 days (40 weeks) from the first day of your last menstrual period. Your doctor confirms this with an ultrasound (which is more accurate early in pregnancy).

Q: Is my due date exact?

A: No. Only 4-5% of babies are born on their due date. Normal is within 2 weeks before or after your due date. Doctors consider pregnancies “overdue” at 42 weeks (2 weeks past due date).

Q: If I’m 8 weeks pregnant, when will I feel the baby move?

A: Most women feel baby movements (quickening) between weeks 16-25. First-time moms often feel it later, experienced moms earlier. Some feel it by week 13-14.

Q: Can I be off about my due date?

A: Yes. Many women miscalculate. Ultrasounds in the first trimester are accurate within 3-5 days. This is why dating ultrasounds are important.

Q: How much should my baby weigh at each week?

A: Baby weight varies widely. Ultrasound can estimate weight but isn’t always accurate. What matters is consistent growth, not exact weight.

Q: Is there a difference between “due date” and “estimated due date”?

A: Not really. Due dates are always estimates. The term “estimated due date” (EDD) is sometimes used to emphasize this, but all due dates are estimates.

Q: What if my last period date is wrong?

A: Tell your doctor. Ultrasound can estimate gestational age based on baby’s size. In first trimester, ultrasound is accurate within ±3 days.

Q: Can I change my due date?

A: Your doctor determines your official due date based on LMP and ultrasound dating. If there’s a discrepancy, ultrasound (especially first trimester) is most accurate.

Tracking Your Weeks: Tools & Apps

Our Pregnancy Calculator

[Link to your calculator page]

• Enter last period date
• Instantly see weeks pregnant and due date

Trusted Pregnancy Apps

• Ovia Pregnancy: Detailed week-by-week info
• Bump: Community + information
• Glow: Tracking + predictions
• Pregnancy+: Comprehensive info

Doctor’s Measurements

• Dating ultrasound (8-14 weeks) most accurate
• Anatomy ultrasound (18-22 weeks) confirms dating
• Third trimester ultrasound less accurate

Timeline: From Week 1 to Birth

Key Takeaways

1. Gestational age = Weeks from first day of last period (what doctors use)
2. Fertilization age = Weeks from conception (about 2 weeks younger)
3. Due date = 280 days from LMP (just an estimate)
4. Ultrasound dating = Most accurate in first trimester
5. Normal delivery = Weeks 37-42 (full term = week 37+)
6. Baby size = Varies normally, not exact

Related Articles

• Early Pregnancy Symptoms Before Missed Period
• Pregnancy Calculator Tool

Conclusion

Calculating how many weeks pregnant you are is simple once you understand that doctors use gestational age (from your last period), not fertilization age.

Your pregnancy is dated from the first day of your last menstrual period. This is medically standard and what your entire pregnancy care is based on.

Use your pregnancy calculator or count forward 280 days from that date to find your due date and current week.

And remember: Due dates are estimates. Babies come on their own schedule, usually within 2 weeks of the due date. You’re not “late” at 41 weeks—you’re just waiting for your baby to be ready.