Your complete week-by-week pregnancy guide
Our pregnancy calculator combines three essential tools in one: estimated due date, gestational weeks, and estimated baby size. Just enter your last menstrual period date to get all the information you need.
How is the due date calculated?
Due Date = Last Menstrual Period + 280 days + Cycle Adjustment
Cycle Adjustment = (Your cycle length - 28) days
Naegele's Rule is the standard method used by obstetricians worldwide.
The three trimesters
First Trimester (Weeks 1-12)
All major organs form during this critical period. By week 12, the baby is about the size of a lime (~14g).
Second Trimester (Weeks 13-26)
Rapid growth phase. The anatomy ultrasound at week 20 evaluates detailed development. The viability milestone is reached at week 24.
Third Trimester (Weeks 27-40)
Final maturation. The baby gains weight rapidly, lungs mature, and the body prepares for birth.
Only 5% of babies are born on their exact due date. Most are born between weeks 37 and 42, which is considered completely normal.
Pregnancy weeks are counted from the first day of your last menstrual period (LMP), not from conception. If your LMP was 10 weeks ago, you're in week 10, even though conception was approximately 2 weeks after your LMP.
Weight varies by week: week 12 (~14g), week 20 (~300g), week 28 (~1kg), week 36 (~2.6kg), week 40 (~3.5kg). These are averages and can vary significantly between healthy babies.
Yes. The standard calculation assumes a 28-day cycle. If your cycle is longer (e.g., 35 days), ovulation occurred later, so extra days are added to the estimated date.
The anatomy ultrasound (weeks 18-22) is the most detailed. It evaluates the baby's entire anatomy and is when many parents learn the sex.
A pregnancy is considered full-term from week 39. Babies born at weeks 37-38 are early-term and usually fine, but each additional week benefits lung and brain development.