Pregnancy Calculator

How this is calculated

This uses Naegele's rule: due date = first day of last period + 280 days (40 weeks), based on an average 28-day cycle.

• Actual cycle length varies between individuals, so an ultrasound-based estimate from a healthcare provider is more precise.

This calculator provides general informational estimates and is not a medical diagnosis. Results should be interpreted alongside other factors. If you have concerns about your health, consider speaking with a qualified healthcare professional.

How pregnancy dates are calculated

Pregnancy is typically dated from the first day of the last menstrual period (LMP), not from the estimated date of conception — even though conception usually occurs about two weeks after LMP. This convention exists because LMP is usually known with more certainty than the exact conception date.

Naegele's Rule (estimated due date)

This assumes a typical 28-day menstrual cycle with ovulation around day 14. For longer or shorter cycles, the estimate can be adjusted accordingly, since the actual conception date shifts along with ovulation timing.

Pregnancy trimesters

Trimester	Weeks	Key milestones
First	1 – 13	Major organ development, highest miscarriage risk period
Second	14 – 27	Often called the most comfortable trimester; anatomy scan typically occurs
Third	28 – 40	Rapid weight gain, final organ maturation, birth preparation

Why due dates are estimates

• Only about 5% of babies are born exactly on their estimated due date.
• Full-term birth is generally considered anywhere from 37 to 42 weeks.
• Ultrasound dating, especially in the first trimester, is often considered more precise than LMP-based dating alone.
• Cycle length variation affects how accurately LMP predicts actual conception timing.

Tips

• If your cycle isn't a typical 28 days, note that the standard formula's accuracy decreases proportionally.
• An early ultrasound can help confirm or adjust the estimated due date.
• Track key milestones by week rather than fixating on the exact due date, since timing naturally varies.

Week-by-week milestone snapshot

Week	Typical milestone
6–8	Heartbeat often detectable via ultrasound
11–14	First-trimester screening / dating scan
18–22	Anatomy scan; fetal sex often determinable
24	Viability milestone (with intensive medical support)
37	Considered full term begins

Frequently asked questions

How do I find what pregnancy week I'm in with this calculator? Enter the first day of your last menstrual period (or conception date), and the calculator counts forward to tell you your current week and trimester automatically.

Why is pregnancy counted from before conception actually happens? LMP is a more reliably known reference point for most people than the exact day of conception, so it became the medical standard for dating.

How accurate is the due date estimate? It's a statistical estimate — most babies are born within a couple of weeks of the estimated date, but exact-day accuracy is uncommon.

What if I don't know my LMP? An early ultrasound measuring fetal size can estimate gestational age and due date independently of LMP.

Why is pregnancy described as 40 weeks even though it's usually about 9 months? 40 weeks (280 days) works out to roughly 9.2 months, and week-based counting gives more precise, standardized tracking than calendar months of uneven length.

What's the difference between gestational age and fetal age? Gestational age counts from LMP (the medical standard), while fetal (conceptional) age counts from actual conception — fetal age is typically about 2 weeks less than gestational age at any given point.

Does an irregular menstrual cycle affect the calculation's accuracy? Yes — this formula assumes a standard 28-day cycle with ovulation on day 14; irregular or notably longer/shorter cycles shift actual conception timing, making an ultrasound-based estimate more reliable.

How is the current week of pregnancy determined? It's simply the number of complete weeks that have passed since the LMP date, calculated the same way regardless of which trimester you're in.

How many weeks pregnant am I based on my due date? Enter your due date instead of your LMP date above (the calculator supports both) — it works backward from the standard 40-week pregnancy length to show your current week automatically.

Another worked example

For an LMP of March 1st with a standard 28-day cycle, the estimated due date falls around December 6th (approximately 280 days later) — an ultrasound in the first trimester can help confirm or adjust this estimate.

What affects the result

• Cycle regularity — due date estimates from LMP assume a standard 28-day cycle; irregular cycles reduce accuracy.
• Ultrasound dating in early pregnancy is generally considered more accurate than LMP-based dating alone.

How to use this calculator

1. Enter the first day of your last menstrual period (LMP).
2. Confirm your typical cycle length if different from 28 days.
3. Press Calculate to see your estimated due date and current week.

Common mistakes to avoid

• Using an inaccurate or estimated LMP date when the actual date is uncertain — this compounds into a less accurate due date.
• Treating irregular cycles the same as regular 28-day cycles without adjustment.
• Assuming the due date is a guaranteed delivery date rather than an estimate.

Key terms explained

LMP: Last Menstrual Period — the standard reference point for dating a pregnancy.

Gestational age: The age of a pregnancy counted from the last menstrual period, not from conception.

Trimester: One of three roughly 13-week periods dividing a pregnancy.