Zwangerschapsweken calculator

How Pregnancy Weeks are Calculated

Pregnancy weeks are counted from the first day of the last menstrual period (LMP). The estimated due date is 280 days (40 weeks) after the LMP. Naegele's rule subtracts 3 months from the LMP date and adds 7 days.

Trimester Breakdown

Weeks 1 through 13 constitute the first trimester, weeks 14 through 27 the second trimester, and weeks 28 through 40 the third trimester. Each period involves different stages of fetal development and maternal health considerations.

How Pregnancy Weeks Are Calculated in Detail

Pregnancy weeks are counted from the first day of the last menstrual period (LMP), even though conception typically occurs about two weeks later. This means weeks one and two of pregnancy predate actual fertilization. An early ultrasound can provide a more accurate gestational age by measuring the embryo. For women with irregular cycles, ultrasound-based dating is generally more reliable than LMP-based calculations.

Key Milestones by Trimester

During the first trimester (weeks 1-13), major organs begin forming and morning sickness is common. The second trimester (weeks 14-27) brings quickening, when fetal movements are first felt, and rapid growth occurs. This is often considered the most comfortable period. The third trimester (weeks 28-40) sees lung maturation, and the mother may experience swelling and back pain as the body prepares for delivery.

When to See a Doctor

Schedule your first prenatal visit before week 8 once pregnancy is confirmed. Seek immediate medical attention for vaginal bleeding, severe abdominal pain, high fever, severe headaches, or vision changes. After week 28, a noticeable decrease in fetal movement can indicate an emergency. Attend all scheduled prenatal checkups and contact your healthcare provider without hesitation if any unusual symptoms arise.