Pregnancy Calculator
Track your pregnancy progress, trimester, and upcoming milestones from your LMP or due date.
Milestones
| Week | Milestone | Date |
|---|---|---|
| Week 8 | Heartbeat detectable | -- |
| Week 12 | End of first trimester | -- |
| Week 20 | Anatomy scan | -- |
| Week 27 | Start of third trimester | -- |
| Week 40 | Estimated due date | -- |
What is Pregnancy Calculator?
A Pregnancy Calculator helps expectant parents track the progress of a pregnancy by estimating gestational age, trimester, and key developmental milestones. It can work from either the first day of the last menstrual period (LMP) or a known due date provided by a healthcare provider.
Knowing your pregnancy timeline helps you prepare for prenatal appointments, anticipate developmental changes, and plan for your baby's arrival. For the single big date on its own, try our Due Date Calculator.
How Gestational Age Is Calculated
By long-standing medical convention, pregnancy is dated from the first day of your last menstrual period, even though conception typically occurs about two weeks later. The core formula is:
Weeks pregnant = (Today's date − First day of LMP) ÷ 7
- Today's date — the day you are checking your progress.
- First day of LMP — the first day of bleeding of your most recent menstrual period.
- The whole-number part is your completed weeks; the remainder is extra days, written as “13 weeks, 6 days.”
The estimated due date comes from Naegele's rule: Due date = LMP + 280 days (40 weeks), equivalent to adding one year, subtracting three months, and adding seven days. The estimated conception date — the likely day of ovulation and fertilization — is LMP + 14 days, matching a standard 28-day cycle. Our Conception Calculator explores that date in more detail.
A Worked Example
Suppose the first day of your last menstrual period was June 1, 2026, and you check the calculator on September 6, 2026:
- Days since LMP: 97, so gestational age is 13 weeks, 6 days (97 ÷ 7 = 13 remainder 6).
- Trimester: Second trimester, because the calculator switches from first to second at day 91 (13 weeks, 0 days).
- Estimated conception: June 15, 2026 (LMP + 14 days).
- Estimated due date: March 8, 2027 (LMP + 280 days, or equivalently June 1, 2026 + 1 year − 3 months + 7 days).
- Days remaining until the due date: 183.
Entering March 8, 2027 in the Due Date tab reproduces every figure above.
Trimester Breakdown
This calculator divides the 40 weeks by day count (some providers quote slightly different cutoffs):
| Trimester | Gestational Age Range | Dates for LMP June 1, 2026 |
|---|---|---|
| First | LMP through 12 weeks 6 days (days 0–90) | June 1 – August 30, 2026 |
| Second | 13 weeks 0 days through 26 weeks 6 days (days 91–188) | August 31 – December 6, 2026 |
| Third | 27 weeks 0 days until delivery (day 189 onward) | December 7, 2026 – due date March 8, 2027 |
Key Pregnancy Milestones
The calculator highlights weeks 8, 12, 20, 27, and 40. The fuller timeline below adds the screening windows most parents ask about, dated for the June 1, 2026 example:
| Timing | Milestone | Date in the Example |
|---|---|---|
| ~Week 6 | Fetal heartbeat may first be detectable on ultrasound | July 13, 2026 |
| Week 8 | Heartbeat reliably detectable (calculator milestone) | July 27, 2026 |
| Weeks 11–14 | Nuchal translucency (NT) scan window | August 17 – September 6, 2026 |
| Weeks 18–22 | Anatomy scan window (calculator highlights week 20) | October 5 – November 8, 2026 |
| ~Week 24 | Viability — survival outside the womb becomes possible with intensive care | November 16, 2026 |
| Week 27 | Third trimester begins | December 7, 2026 |
| Week 39 | Full term | March 1, 2027 |
| Week 40 | Estimated due date | March 8, 2027 |
The 28-Day Cycle Assumption
Naegele's rule assumes a textbook 28-day cycle with ovulation on day 14. If your cycles are reliably longer or shorter, ovulation — and therefore your true conception and due dates — shifts by the same number of days:
- Longer cycle: add the extra days. A 35-day cycle pushes the example due date to March 15, 2027 (287 days after the LMP).
- Shorter cycle: subtract the difference. A 24-day cycle moves the due date earlier to March 4, 2027.
Our Ovulation Calculator and Period Calculator can help you pin down your typical cycle length before relying on LMP-based dating.
Why an Ultrasound May Revise Your Due Date
An early ultrasound is the most accurate way to date a pregnancy: in the first trimester, measuring the embryo's crown-rump length predicts gestational age to within about five to seven days. If that measurement disagrees with LMP-based dating by more than roughly a week — common when cycles are irregular or the LMP is uncertain — your provider will often re-date the pregnancy from the ultrasound. Later ultrasounds are progressively less precise, so providers avoid changing a due date late in pregnancy unless the discrepancy is large.
This pregnancy calculator provides general educational estimates only and is not medical advice. Your healthcare provider confirms your dates and care plan through clinical evaluation and ultrasound.
How to Use
- Choose whether to calculate from your Last Menstrual Period or a known Due Date.
- Enter the appropriate date in the date picker.
- Click Calculate to see gestational age, trimester, days remaining, and estimated conception date.
- Review the milestones table for key dates during your pregnancy.
Frequently Asked Questions
How is gestational age calculated?
Gestational age is counted from the first day of your last menstrual period (LMP), not from conception, which usually occurs about two weeks later. Divide the days since your LMP by seven to get weeks pregnant.
How does this pregnancy calculator estimate my due date?
It applies Naegele's rule: your due date equals the first day of your last menstrual period plus 280 days (40 weeks), assuming a 28-day cycle with ovulation on day 14. You can also work backward from a known due date to find your implied LMP and current gestational age.
When does each trimester start and end?
Following the day-count convention used by this calculator, the first trimester runs from the LMP through 12 weeks 6 days, the second trimester spans 13 weeks 0 days through 26 weeks 6 days, and the third trimester begins at 27 weeks and lasts until delivery.
What if my cycle is irregular?
An irregular cycle can affect accuracy because the 280-day rule assumes ovulation on day 14 of a 28-day cycle. If your cycle is consistently longer, add the extra days to your due date; if shorter, subtract them. When cycle length varies significantly, early ultrasound dating is the most reliable method.
Is the due date guaranteed?
No. Only about 5% of babies are born exactly on their due date. A normal, full-term delivery can occur anywhere from 37 to 42 weeks, so treat the due date as the midpoint of a likely window rather than a fixed appointment.
What if I don't know my last menstrual period?
Use the Due Date tab and enter the due date given by your provider, or ask about an early ultrasound. A first-trimester ultrasound that measures the embryo can date a pregnancy to within about five to seven days.
Is this calculator a substitute for medical advice?
No. This tool offers general educational estimates only and cannot account for your individual health history. Your healthcare provider confirms your dating and prenatal care plan, so always follow their guidance.