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About This Calculator
Pregnancy is medically tracked in weeks rather than months because fetal development milestones are tied to specific weekly intervals. Knowing your exact week helps your healthcare provider schedule appropriate tests, screenings, and ultrasounds at the right time. This calculator determines your current pregnancy week from your last menstrual period or conception date and highlights what developmental stage your baby has reached.
Quick Tips
- 1 Pregnancy is counted from the last menstrual period, not from conception date.
- 2 Each trimester spans roughly 13 weeks: 1-13, 14-26, and 27-40.
- 3 Full term is 39-40 weeks; babies born at 37-38 weeks are early term.
Example Calculation
Last menstrual period: December 29, 2025, today March 17, 2026.
Gestational age: 11 weeks 1 day | Trimester: First | Due: October 5, 2026 | Baby size: ~1.6 inches
How Pregnancy Weeks Are Counted
Pregnancy is counted from the first day of your last menstrual period (LMP), not from conception. This means you are technically "pregnant" for about two weeks before conception actually occurs. A full-term pregnancy is 40 weeks (280 days) from the LMP. This standard dating system is used worldwide by obstetricians and midwives to track fetal development and schedule prenatal care.
Understanding Trimesters
Pregnancy is divided into three trimesters. The first trimester (weeks 1-12) involves rapid cell division, organ formation, and the highest risk of miscarriage. The second trimester (weeks 13-27) is often called the "golden period" as morning sickness usually subsides and the baby grows significantly. The third trimester (weeks 28-40) involves final organ maturation, weight gain, and preparation for birth.
Baby Size Milestones
Baby size comparisons help visualize growth: at 8 weeks the embryo is the size of a raspberry, at 12 weeks a lime, at 20 weeks a banana, at 24 weeks an ear of corn, at 28 weeks an eggplant, at 32 weeks a squash, at 36 weeks a honeydew melon, and at 40 weeks a small watermelon. These comparisons are approximate and based on average fetal measurements.
Frequently Asked Questions
The exact date of conception is usually unknown because sperm can survive up to 5 days in the reproductive tract. The first day of the last menstrual period is a reliable, observable date that most women can identify. Ovulation typically occurs around day 14 of a 28-day cycle, so gestational age is approximately 2 weeks longer than the actual fetal age.
The estimated due date (EDD) is 40 weeks (280 days) from the first day of the last menstrual period. Only about 5% of babies are born on their exact due date. Most healthy births occur between weeks 37 and 42. First-time mothers tend to deliver slightly past their due date on average.
The first trimester spans weeks 1 through 12, the second trimester weeks 13 through 27, and the third trimester weeks 28 through 40. Some sources define slightly different boundaries, but these are the most commonly used divisions in clinical practice.
At 20 weeks (the halfway point), the baby is approximately 6.5 inches long from crown to rump and weighs about 10.6 ounces. This is roughly the size of a banana. By this stage, the anatomy scan can reveal the sex, and the mother typically begins feeling fetal movements (quickening).