Due Date Calculator — Spain
Your estimated due date is calculated using Naegele's rule: add 280 days (40 weeks) to the first day of your last menstrual period, adjusted for cycle length. For example, if your last period started January 1, 2026 with a 28-day cycle, your estimated due date would be October 8, 2026. Only about 5% of babies are born on their exact due date — most arrive between weeks 38 and 42. This calculator shows your estimated date, current gestational week, and important pregnancy milestones.
Frequently Asked Questions
How accurate is the calculated due date?
The estimated due date is based on Naegele's rule. Only about 5% of babies are born on their exact due date. Most births occur between weeks 38 and 42 of gestation. A first-trimester ultrasound can provide a more precise estimate if there is a discrepancy with the date based on the last menstrual period.
How is the due date calculated using Naegele's rule?
Naegele's rule adds 280 days (40 weeks) to the first day of the last menstrual period (LMP). If your cycle is not 28 days, the calculation adjusts by adding or subtracting the difference. For example, with a 30-day cycle, 282 days are added. This method assumes ovulation on day 14 of the cycle, so the adjustment compensates for shorter or longer cycles.
What is gestational age and how is it counted?
Gestational age is counted from the first day of the last menstrual period, not from conception. This means the first two weeks of pregnancy are before ovulation and fertilization. When your doctor says you are 8 weeks pregnant, the embryo is approximately 6 weeks old. It is expressed in weeks and days (for example, 12+3 means 12 weeks and 3 days).
What are the trimesters of pregnancy?
Pregnancy is divided into three trimesters: the first trimester runs from week 1 to 12 (organ formation), the second trimester from week 13 to 26 (rapid growth, first movements), and the third trimester from week 27 to 40 (lung maturation, preparation for birth). Each trimester has specific medical checkups and tests.
What should I do if my menstrual cycle is irregular?
If your cycle is irregular, the calculated date may be less accurate. In this case, a first-trimester ultrasound (between weeks 8 and 12) is the most reliable method for determining gestational age and estimated due date. Consult your doctor for a more precise estimate based on embryonic ultrasound measurements.