Pregnancy Calculator

What does a pregnancy calculator tell you?

A pregnancy calculator answers the first question every expectant mother asks: how far along am I? Enter the first day of your last menstrual period (LMP) and it tells you your gestational age in weeks and days, which trimester you are in, and your estimated due date. These three numbers shape almost everything that follows — which scans are due, which tests are offered, and what to expect each week.

Pregnancy is dated from the LMP rather than from conception, because the LMP is a date most women can recall while the exact day of conception usually cannot be known. This is the standard doctors and midwives use worldwide.

How it works

• Gestational age = days elapsed since the first day of your last period, expressed as weeks + days (e.g. 14w 2d).
• Due date = LMP + 280 days (40 weeks), known as Naegele's rule.
• Trimester = first: days 0–90 (weeks 0–13), second: days 91–188 (weeks 14–27), third: day 189 onward (week 28+).

The 280-day figure assumes a regular 28-day cycle with ovulation around day 14 — the two "extra" weeks before conception are simply how medical dating works.

How to use this calculator

1. Enter the first day of your last menstrual period.
2. Read your current week and day, your trimester, and your due date.
3. Use the trimester to know what is typical now: first-trimester screening, the week-20 anomaly scan, third-trimester growth checks.

Worked example

If your last period began 100 days ago:

• Gestational age = 100 ÷ 7 = 14 weeks and 2 days
• Trimester: 100 days falls in the 91–188 band → second trimester
• Due date = LMP + 280 days → 180 days from today

At 14w 2d you are just into the second trimester — often when early symptoms ease and energy returns.

Limitations and tips

• Irregular cycles: if your cycles are much shorter or longer than 28 days, LMP dating can be off by a week or more. An early ultrasound (dating scan) is more reliable.
• Only ~5% arrive on the due date: most babies come within two weeks either side. Treat the date as the middle of a window, not an appointment.
• General information only: this calculator cannot replace antenatal care. If you have bleeding, severe pain, or any worry, contact your doctor or midwife promptly.
• Track weekly: save your LMP date somewhere safe — you will be asked for it at nearly every appointment.