How Pregnancy Dating Works
Pregnancy is most commonly measured from the first day of your last menstrual period (LMP), even though conception typically occurs about two weeks later. This convention is used because the LMP date is usually known with certainty, while the exact date of ovulation and conception is harder to pin down.
Using the LMP method, a full-term pregnancy lasts 280 days (40 weeks). If you know your conception date instead, you can use the "From Conception" tab — pregnancy from conception is 266 days (38 weeks).
Understanding Your Due Date
- The estimated due date (EDD) is calculated as LMP + 280 days (or conception + 266 days).
- Only about 5% of babies are born exactly on their due date.
- A normal delivery window is between 37 and 42 weeks.
- Your healthcare provider may adjust your due date based on ultrasound measurements.
The Three Trimesters
Pregnancy is divided into three trimesters, each with distinct developmental milestones:
- First Trimester (weeks 0–13): All major organs begin to form. Miscarriage risk is highest. Many women experience morning sickness.
- Second Trimester (weeks 14–27): The fetus grows rapidly. The anatomy scan at around week 20 checks for abnormalities. Many women feel the baby move for the first time.
- Third Trimester (weeks 28–40+): Final growth and lung development. The baby moves into position for birth. Braxton Hicks contractions may begin.
Frequently Asked Questions
What is gestational age?
Gestational age is the number of weeks pregnant you are, counted from the first day of your last menstrual period. It is the standard way doctors and midwives describe how far along a pregnancy is. Fetal age — counted from conception — is typically about two weeks less than gestational age.
How accurate is this calculator?
This calculator gives an estimate based on the date you enter. It is intended for informational purposes only. For accurate pregnancy dating, your healthcare provider will typically perform an ultrasound, which is considered the gold standard, especially in the first trimester.
What is the viability milestone at 24 weeks?
Fetal viability refers to the point at which a baby born prematurely has a reasonable chance of survival with medical support. This threshold is generally considered to be around 22–24 weeks of gestational age, with survival rates improving significantly with each additional week.