Pregnancy Week Calculator: How Many Weeks Pregnant Am I?
This pregnancy week calculator tells you exactly how many weeks and days pregnant you are today, plus your trimester, months, due date and days left. Use your last period, due date or IVF transfer date. Private — runs in your browser.
A pregnancy week calculator tells you your gestational age: how many completed weeks and days pregnant you are, counted from the first day of your last menstrual period (LMP). From that date it works out your trimester, roughly how many months along you are, and your estimated due date, which is LMP plus 280 days (40 weeks).
How the Pregnancy Week Calculator Works
Pregnancy is counted from the first day of your last menstrual period, not from conception, so the count starts about two weeks before the egg is fertilized. If you only know another date, the calculator converts it to an LMP-equivalent date first, then counts the days to today.
Dating formulas used
From LMP: due date = LMP + 280 days (Naegele's rule)
From a known due date: LMP-equivalent = due date − 280 days
From conception: due date = conception + 266 days (LMP-equivalent = conception − 14 days)
IVF, day-5 embryo: due date = transfer date + 261 days
IVF, day-3 embryo: due date = transfer date + 263 days
Weeks pregnant = whole days since LMP ÷ 7 (remainder = extra days)
The IVF offsets come from ACOG Committee Opinion 700, Methods for Estimating the Due Date, which says a pregnancy conceived with assisted reproduction should be dated from the embryo age and transfer date. Days are counted calendar-day to calendar-day, so daylight-saving changes never shift the result.
“18 Weeks 1 Day” vs “Week 19”: Reading Your Pregnancy Week
Doctors write gestational age as completed weeks plus days, for example 18w1d. That means 18 full weeks have passed and you are now in your 19th week. Both phrases describe the same day; “18 weeks pregnant” and “in week 19” are not a disagreement. This tool always shows both.
| Worked example (today = Oct 6, 2026) | Input | Result |
|---|---|---|
| From last period | LMP June 1, 2026 | 127 days = 18 weeks 1 day pregnant (in week 19), 2nd trimester, about 4.2 months, due Mar 8, 2027, 153 days to go |
| From IVF day-5 transfer | Transfer May 20, 2026 | LMP-equivalent May 1, 2026 → 22 weeks 4 days (in week 23), due Feb 5, 2027 (transfer + 261 days) |
Weeks to Months Pregnant: Conversion Table
There is no single official way to turn pregnancy weeks into months, because calendar months are 28 to 31 days long while pregnancy is tracked in weeks. Clinicians use weeks and trimesters. The table below converts weeks to calendar months elapsed since your LMP (weeks × 7 ÷ 30.44), alongside the trimester boundaries used by ACOG: first trimester through 13 weeks 6 days, second 14w0d to 27w6d, third from 28w0d (ACOG: How Your Fetus Grows During Pregnancy).
| Weeks pregnant | Calendar months since LMP | Trimester |
|---|---|---|
| 4 weeks | ≈ 0.9 months | 1st |
| 8 weeks | ≈ 1.8 months | 1st |
| 13 weeks | ≈ 3.0 months | 1st (ends at 13w6d) |
| 14 weeks | ≈ 3.2 months | 2nd begins |
| 22 weeks | ≈ 5.1 months | 2nd |
| 26 weeks | ≈ 6.0 months | 2nd (ends at 27w6d) |
| 28 weeks | ≈ 6.4 months | 3rd begins |
| 35 weeks | ≈ 8.0 months | 3rd |
| 40 weeks | ≈ 9.2 months | 3rd (due date) |
Pregnancy Week by Week: Key Milestones
Sizes are approximate averages reported by ACOG for the end of each week; individual pregnancies vary.
| End of week | Milestone |
|---|---|
| 8 | Embryo about half an inch long; arm and leg buds, face and digestive tract forming |
| 10 → 11 | Fetal stage begins at week 11 |
| 12 | About 2 inches long, about half an ounce |
| 13 | All major organs have formed |
| 20 | More than 6 inches long, under 11 ounces |
| 24 | About 12 inches, about 1½ pounds |
| 28 | Nearly 15 inches, about 2½ pounds |
| 32 | Almost 17 inches, a little over 4 pounds |
| 36 | About 18 inches, a little over 6 pounds; most turn head-down around week 36–37 |
| 40 | About 20 inches, may weigh 7½ to 8 pounds |
Source: ACOG, How Your Fetus Grows During Pregnancy. ACOG classes births from 37w0d to 38w6d as early term, 39w0d to 40w6d as full term, 41w0d to 41w6d as late term, and 42w0d onward as post-term (ACOG Committee Opinion 579).
Tips for an Accurate Pregnancy Week
LMP dating assumes a regular cycle with ovulation about two weeks after your period starts. If your cycles are long, irregular, or you are unsure of the date, an early ultrasound is usually more reliable, and your provider may change your due date after it. Once a due date is set by your provider, choose “Known due date” above so this calculator matches the weeks they use. For IVF pregnancies, use the transfer date and embryo age.
This calculator is for general information only and does not replace medical advice. Confirm your dates with your doctor or midwife.
Last updated: October 2026
Frequently Asked Questions
How are pregnancy weeks calculated?
Pregnancy weeks are counted from the first day of your last menstrual period (LMP). This means "Week 1" starts before conception. Actual fertilization usually occurs around Week 2. This is standard medical practice worldwide.
How many weeks pregnant am I?
Count the days from the first day of your last period to today and divide by 7: the whole number is completed weeks and the remainder is days. For example, 127 days is 18 weeks 1 day pregnant, which means you are in your 19th week. If you know your due date, subtract 280 days to get the start date, then count the same way.
How do I convert pregnancy weeks to months?
There is no single official conversion, because months vary in length. A simple method is weeks x 7 / 30.44, which gives calendar months since your last period: 13 weeks is about 3.0 months, 26 weeks about 6.0 months and 40 weeks about 9.2 months. Doctors track pregnancy in weeks and trimesters instead.
How accurate is the due date?
The due date is an estimate — only about 4-5% of babies arrive on their exact due date. ACOG classes births from 37 weeks 0 days through 41 weeks 6 days as early, full or late term. Your doctor may adjust the due date based on ultrasound measurements.
What trimester am I in?
Per ACOG, the 1st trimester runs from the first day of your last period through 13 weeks 6 days, the 2nd trimester from 14 weeks 0 days through 27 weeks 6 days, and the 3rd trimester from 28 weeks 0 days until birth.
When will I feel the baby kick?
Most first-time mothers feel movement (quickening) between weeks 18-25. Experienced mothers may feel it as early as week 16. Early movements feel like fluttering or bubbles.
Is my data private?
Yes. All calculations run entirely in your browser. No pregnancy data is sent to any server. Your health information is completely private.