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Pregnancy Calculator

Calculate your due date, conception date, and current pregnancy week. Track fetal development milestones week by week. Enter your last period or conception date to get instant results.

How Due Date Is Calculated

Naegele's rule is the standard medical method for estimating due dates. This calculator uses it to add 280 days (40 weeks) to the first day of your last menstrual period (LMP).

Pregnancy weeks are counted from the LMP, not conception. This means at conception, you're already considered 2 weeks pregnant. The calculation assumes a 28-day menstrual cycle with ovulation on day 14.

First Trimester

Weeks 1-13. Foundation period with major organ development.

Second Trimester

Weeks 14-27. Growth period with movement and development.

Third Trimester

Weeks 28-40+. Preparation period for birth and maturation.

Last Period vs. Conception Date: Which Should You Use?

Switch between the two tabs depending on what you know. If you track your cycle, use From Last Period and enter the first day of your LMP; the calculator adds 280 days to get your due date. If you know exactly when you conceived (for example, from an ovulation test or IVF transfer), use From Conception instead, which adds 266 days (38 weeks) to that date, the medically equivalent calculation.

This is why pregnancy is dated in a way many people find confusing: weeks are counted from the LMP, not from conception. Since ovulation and conception happen around day 14 of an average cycle, you are already considered "2 weeks pregnant" on the day you actually conceive. Under the hood, the From Conception mode simply subtracts 14 days from your entered date to derive an equivalent LMP, then runs the same calculation.

How Your Current Week and Trimester Are Determined

The calculator counts the full days between your LMP and today, divides by seven, and adds one to get your current week; the remainder gives your current day within that week. Your trimester is then set directly from that week number: weeks 1-13 fall in the first trimester, weeks 14-27 in the second, and week 28 onward in the third. Days remaining until your due date are calculated the same way, counting forward from today to your due date.

Because the whole calculation is date arithmetic, small differences in cycle length compound over the pregnancy. A shorter-than-average cycle (fewer than 28 days) means ovulation happened earlier than day 14, so your true due date is likely earlier than Naegele's rule suggests; a longer cycle pushes it later. This is normal and is one reason clinicians re-date pregnancies after an early ultrasound.

Week-by-Week Fetal Development

Your baby grows from a poppy seed to a full-term baby over 40 weeks. Entering your LMP or conception date shows your current week with a size comparison (fruit or vegetable), estimated weight and length, and the key milestones typical for that week.

Weeks 1-13

The heart starts beating around week 4-5, limb buds and facial features appear, and by week 12-13 all major organs have formed and fingerprints are present.

Weeks 14-27

Movement, hearing, and hair growth develop; by week 20 you are halfway through, and by week 27 the eyes have opened and the brain is increasingly active.

Weeks 28-40+

Rapid weight gain, lung maturation, and practice breathing movements prepare the baby for birth; term begins at 37 weeks, though the exact stage of term matters (see below).

Term Isn't One Date: Early, Full, Late, and Postterm

"Full term" used to describe any birth from 37 weeks to 42 weeks, but research showing that outcomes, especially newborn breathing problems, vary across that range led the American College of Obstetricians and Gynecologists to split it into four categories in 2013.

Early term: 37 0/7 to 38 6/7 weeks

Considered term, but babies born in this window have a slightly higher rate of complications than full term.

Full term: 39 0/7 to 40 6/7 weeks

The window with the lowest rate of newborn complications, and the target range for scheduled inductions without a medical reason.

Late term: 41 0/7 to 41 6/7 weeks

Many providers begin discussing induction in this window, since the risk of complications rises somewhat past the due date.

Postterm: 42 0/7 weeks and beyond

Associated with higher risk and is usually actively managed rather than allowed to continue without intervention.

Important Note

This calculator provides estimates based on average cycles. Only about 5% of babies are born on their exact due date. Always consult your healthcare provider for accurate due date determination and prenatal care. Early ultrasound measurements provide more precise dating than LMP alone.

Frequently Asked Questions

How is the due date calculated?▾
The due date is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period (LMP). This assumes a 28-day cycle with ovulation on day 14. The calculation is shown instantly when you enter your LMP or conception date.
How accurate are due dates?▾
Only about 5% of babies are born on their exact due date. Most full-term births occur between 37 and 42 weeks. Due dates are estimates based on average cycle length. Your healthcare provider may adjust your due date based on early ultrasound measurements, which are more accurate than LMP-based calculations.
What is Naegele's rule?▾
Naegele's rule is a standard method for estimating due date: start with the first day of your last period, add one year, subtract three months, and add seven days. Alternatively, add 280 days to your LMP. This method was developed in the 1800s and remains the standard calculation method today.
Can I calculate from conception date?▾
Yes, switch to "From Conception" mode and enter the date you conceived. The calculator will estimate your due date by adding 266 days (38 weeks) to the conception date. Conception typically occurs 12-24 hours after ovulation, which is usually around day 14 of a 28-day menstrual cycle.
What week does each trimester start?▾
The first trimester covers weeks 1-13 (months 1-3), the second trimester covers weeks 14-27 (months 4-6), and the third trimester covers weeks 28-40+ (months 7-9). Each trimester brings different developmental milestones and changes for both mother and baby.
How do I know my current week?▾
Enter the first day of your last period. The calculator shows your current week and day. Pregnancy weeks are counted from the first day of your LMP, not from conception. For example, if you're 8 weeks pregnant, conception occurred about 6 weeks ago.
What if my cycle isn't 28 days?▾
Naegele's rule assumes a 28-day cycle, but many women have shorter or longer cycles. If your cycle is shorter than 28 days, your due date may be earlier; if longer, it may be later. Healthcare providers typically adjust due dates based on early ultrasound measurements for better accuracy.
What is the difference between full term and just "term"?▾
Since 2013, the American College of Obstetricians and Gynecologists has split what used to be called "full term" (37-42 weeks) into four categories: early term (37 0/7 to 38 6/7 weeks), full term (39 0/7 to 40 6/7 weeks), late term (41 0/7 to 41 6/7 weeks), and postterm (42 0/7 weeks and beyond). The change reflects research showing newborn outcomes differ across that range, so a baby born at 37 weeks is term but not yet at the lowest-risk full term window.
When will I feel the baby move?▾
First-time mothers typically feel initial movement, called quickening, around 18-20 weeks, while those who have been pregnant before often notice it earlier, around 16-18 weeks, since they recognize the sensation. Early movement can feel like flutters or gas bubbles before becoming more distinct kicks later in the second trimester. If movement patterns change noticeably or stop, contact a healthcare provider promptly rather than waiting for a scheduled appointment.

By Toolember · Updated September 2026