Pregnancy Calculator

How to use this calculator

1. Choose how you are dating the pregnancy: Due Date, Last Period, Ultrasound, Conception Date or IVF Transfer Date. Only that method's fields appear.
2. Fill them in. Under Last Period you can also give your average cycle length if it is not the textbook 28 days.
3. Press Calculate Pregnancy Progress to see how many weeks and days along you are today, your trimester, a progress bar toward your due date, and the whole schedule with today's position marked in it.

The five ways to date a pregnancy

Four of these estimate the due date; the fifth starts from one you already have. They all converge on the same thing — a last menstrual period, from which every other date follows — but they differ in how much guesswork stands in the way, so they are not equally certain.

• Due Date is for when your provider has already given you one, often revised after a dating scan. The calculator runs the 280-day rule backwards and builds the schedule from there, so nothing here contradicts what you were told.
• Last Period is the conventional route and the one prenatal care starts from, because the LMP is a date most people can name. It assumes ovulation around day 14, which is why a cycle that is not 28 days shifts everything.
• Ultrasound is the most reliable estimate early on, because the scan measures the pregnancy rather than inferring it from a cycle.
• Conception Date counts 266 days forward instead of 280 — the same rule, started two weeks later. Useful if you know the date, which most people do not.
• IVF Transfer Date is the most precise of the five: the embryo's age on transfer day is known exactly, so a 5-day transfer is due 261 days later and a 3-day transfer 263.

How pregnancy is dated

By long-standing convention, pregnancy is counted from the first day of your last period — about two weeks before conception actually occurs. The LMP is used because it is a date most people can pinpoint, while the exact moment of conception usually cannot be known. Every prenatal test and milestone is scheduled against this gestational timeline.

Gestational age versus fetal age

Gestational age is measured from the LMP and is the standard used in prenatal care. Fetal age (or conceptional age) is counted from conception and runs about two weeks less. So "40 weeks pregnant" by gestational age is roughly 38 weeks since conception — the same pregnancy, two different starting lines.

The key dates

All the dates derive from your LMP:

• Due date = LMP + 280 days (Naegele's rule)
• Estimated conception = LMP + 14 days
• End of the first trimester = end of week 13 (LMP + 91 days)
• End of the second trimester = end of week 27 (LMP + 189 days)

A worked example

Say your last period began on January 1, 2026. Conception is estimated around January 15, the first trimester ends about April 2, the second ends about July 9, and your due date is October 8, 2026. If today were June 1, you would be 21 weeks and 4 days along — in the second trimester, just past the halfway mark.

The three trimesters

• First (weeks 1–13): major organs form; early symptoms and the highest rate of developmental change.
• Second (weeks 14–27): often the most comfortable stretch, and when movement usually becomes noticeable.
• Third (weeks 28–birth): rapid growth and preparation for delivery.

What full term means

Full term is 39–40 weeks, and birth any time from 37 to 42 weeks is considered normal, so the 40-week due date is the midpoint of a range rather than a deadline. Only about 4% of babies arrive on the exact date. For the due date on its own, use the due date calculator.

Everything here is educational. Your healthcare provider tracks your pregnancy with examinations and ultrasounds and can adjust these dates to your individual situation — and if they have already given you a due date, enter that one and the schedule will follow it.