Pregnancy Week Calculator

How Pregnancy Weeks Are Calculated

Pregnancy is typically dated from the first day of your last menstrual period (LMP), not from the date of conception. This may seem counterintuitive, but it's the standard method used by healthcare providers worldwide because:

• Most women know when their last period started
• The exact date of conception is often unknown
• It provides a consistent starting point for all pregnancies

This means that when you conceive (about 2 weeks after your LMP), you're already considered "2 weeks pregnant."

The Three Trimesters

Important Pregnancy Milestones

• Week 4-5: Pregnancy test becomes positive
• Week 6-7: Heartbeat can be detected on ultrasound
• Week 12: End of first trimester, reduced miscarriage risk
• Week 16-20: You may feel first movements (quickening)
• Week 18-22: Anatomy scan ultrasound
• Week 24: Viability milestone - baby could survive if born early
• Week 37: Considered full term
• Week 40: Due date

Due Date Accuracy

Only about 5% of babies are born on their exact due date. Most babies arrive within two weeks before or after the due date. Your healthcare provider may adjust your due date based on:

• Early ultrasound measurements (most accurate)
• Physical examination findings
• IVF transfer date (if applicable)

A pregnancy is considered full-term between weeks 39 and 40, early-term between weeks 37 and 38, and post-term after week 42.

When to See Your Healthcare Provider

• As soon as you have a positive pregnancy test
• If you experience bleeding or severe cramping
• If you have concerns about your symptoms
• For regular prenatal checkups as recommended

Worked Example: Dating a Pregnancy from Your LMP

Suppose the first day of your last menstrual period (LMP) was March 10. Standard dating (Naegele's rule) adds 280 days — exactly 40 weeks — to that date, giving an estimated due date of December 15. If today is May 5, you are 8 weeks and 0 days pregnant, in the first trimester, with 32 weeks remaining.

The calculator handles three starting points and converts each to the same 40-week timeline:

• Last period: gestational age = days since LMP ÷ 7. This is what your doctor and ultrasound reports use.
• Conception date: conception happens roughly at ovulation, about 2 weeks after the LMP, so the tool adds 14 days first. A known conception date of April 1 corresponds to a gestational start of March 18.
• Due date: working backward, LMP = due date − 280 days. This is handy if your provider already gave you a due date and you want to know your current week.

Note that "8 weeks pregnant" means you have completed 8 full weeks — you are actually in your ninth week of pregnancy. Weeks are always counted as completed weeks plus days (e.g. "8w3d" on an ultrasound report means 8 weeks, 3 days).