DUE DATE TOOL

Pregnancy Countdown

Enter your last period or due date to see exactly where you are — and when to expect your little one.

A pregnancy due date is calculated by adding 280 days (40 weeks) to the first day of your last menstrual period using Naegele's rule. This free calculator shows your due date, current week and trimester, days remaining, and a live countdown — all privately in your browser.

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What Happens in Each Trimester?

Trimester 1
Weeks 1 – 12
💓
First Heartbeat
~Week 6
🔬
First Ultrasound
~Week 8
✅
End of First Trimester
Week 12
Trimester 2
Weeks 13 – 26
👶
Gender Reveal Possible
~Weeks 18 – 20
🦶
Baby Kicks
~Week 20
⭐
Viability Milestone
~Week 24
Trimester 3
Weeks 27 – 40
🎉
Baby Shower Window
~Weeks 28 – 34
🏥
Early Term
~Week 37
🎂
Due Date
~Week 40

How Is a Pregnancy Due Date Calculated?

A pregnancy due date is most commonly estimated using Naegele's rule: take the first day of your last menstrual period (LMP) and add 280 days (40 weeks). This method, established by German obstetrician Franz Naegele in 1812, assumes a regular 28-day menstrual cycle with ovulation occurring around day 14. It remains the standard starting point used by healthcare providers worldwide.

However, Naegele's rule is an approximation. According to a 2013 study published in Human Reproduction (Jukic et al.), the median length of a natural pregnancy from ovulation to birth is 268 days, with a standard deviation of about 10 days. When measured from the last menstrual period, variability is even wider (SD of about 14 days) because of differences in ovulation timing. Only roughly 4–5% of babies arrive on the exact estimated due date. The normal delivery window spans 37 to 42 weeks of gestation, meaning the actual birth date can fall within a window of about five weeks.

Several factors influence when delivery actually occurs: menstrual cycle length (longer cycles push the date later), maternal age (first-time mothers over 35 tend to carry slightly longer), parity (some population studies suggest first pregnancies last roughly one to two days longer than subsequent ones, though results vary), and individual biological variation. An early ultrasound performed between 8 and 13 weeks can refine the due date to within ±5 to 7 days by measuring the embryo's crown-rump length, and providers will typically adjust the LMP-based date if the ultrasound differs by more than 7 days.

How Do You Use This Pregnancy Countdown Calculator?

Using this tool takes three steps. First, choose your input mode — either "I know my LMP" if you have the date of your last menstrual period, or "I know my due date" if your provider has already given you an estimated due date. Second, enter the date in the field provided. Third, press "Calculate." The tool instantly displays your due date, current gestational week and day, which trimester you are in, a visual progress bar, and a live countdown timer — all calculated privately in your browser with no data sent to any server.

Pregnancy Due Date Questions & Answers

Naegele's rule provides a reasonable estimate for people with regular 28-day cycles, but individual variation is significant. Population studies show that the average pregnancy lasts about 281–282 days from LMP, with a standard deviation of roughly 13–14 days. An early ultrasound can improve accuracy considerably, which is why providers adjust the due date when ultrasound measurements differ from the LMP-based estimate by more than 7 days.
The first trimester covers weeks 1 through 12, the second trimester runs from week 13 through week 26, and the third trimester spans week 27 through week 40 (or delivery). These divisions are approximate — some sources draw the boundaries a week earlier or later — but this breakdown is the most commonly used in clinical practice.
If you are unsure of your last menstrual period date, your healthcare provider can estimate your due date using an early pregnancy ultrasound. Measurements of the embryo — particularly crown-rump length before 14 weeks — provide a reliable alternative for dating. You can also use this calculator's "I know my due date" mode once your provider gives you an estimated date.
Yes. If an ultrasound performed in the first trimester shows that the baby's size differs from the LMP-based estimate by more than about 7 days, your provider will typically adjust the due date. Later ultrasounds are less accurate for dating because babies grow at different rates. A revised due date does not mean anything is wrong — it simply refines the original estimate.
A pregnancy is considered "early term" at 37 weeks, "full term" between 39 and 40 weeks, "late term" at 41 weeks, and "post-term" at 42 weeks and beyond. The 40-week mark (280 days from LMP) is the traditional due date, but healthy babies routinely arrive anywhere from 37 to 42 weeks of gestation.
Only about 4–5% of babies arrive on the exact estimated due date. Most babies are born within a window of two weeks before or after the due date. A 2013 study in Human Reproduction found that natural pregnancy length varies by as much as 37 days between individuals, making the due date a midpoint estimate rather than a precise prediction.
To calculate your due date from your last menstrual period, add 280 days (or 40 weeks) to the first day of your most recent period. This is known as Naegele's rule. For example, if your last period started on January 1, your estimated due date would be October 8. This calculator performs that math for you automatically.
Yes. Naegele's rule assumes a 28-day cycle. If your cycle is consistently longer, such as 35 days, you likely ovulated later, and your actual due date may be about a week later than the standard calculation. Conversely, shorter cycles may mean an earlier due date. Your healthcare provider can adjust the estimate if your cycle length differs significantly from 28 days.
A pregnancy is classified as "late term" at 41 weeks and "post-term" at 42 weeks and beyond. Most providers will discuss induction options between 41 and 42 weeks because the risk of complications increases after that point. In modern obstetric practice, fewer than 5–6% of pregnancies reach 42 weeks, largely because induction is routinely offered by 41 weeks.
No. An online due date calculator gives you a useful estimate based on Naegele's rule, but it cannot account for individual factors like irregular cycles, health conditions, or ultrasound findings. Always consult your healthcare provider for an accurate due date and comprehensive prenatal care tailored to your specific pregnancy.
Medical Disclaimer
This tool is for informational purposes only and is not medical advice. Consult your healthcare provider for an accurate due date and prenatal care.