What is the pregnancy week (gestational age)?
Counting method, meaning and medical use
The pregnancy week (gestational age) is the most important measure of how a pregnancy is progressing. In Germany and most other countries it is counted from the first day of the last menstrual period (LMP), not from the moment of conception. A normal pregnancy spans 40 weeks (280 days), from week 0+0 to week 40+0.
The notation commonly used in Germany and in medicine, "week X+Y", states how many complete weeks have passed (X) and how many days of the current week have already elapsed (Y, 0 to 6). "12+3" means: 12 complete weeks have passed, and you are on the third day of week 13. In English-speaking countries the phrasing "12 weeks and 3 days" is also common.
The pregnancy week serves as the basis for prenatal care in Germany: the maternity guidelines prescribe check-ups at specific weeks (e.g. first-trimester screening around week 11–13, anomaly scan around week 19–22, third screening around week 29–32). If the doctor does not know the current week, these windows can be missed.
If the LMP is unclear or cycles are irregular, an ultrasound examination is the more accurate method for determining the pregnancy week. In the first trimester, the crown-rump length of the embryo can indicate developmental stage to within a few days. From the second trimester onward, the ultrasound estimate becomes less precise, and the calculated week (based on the LMP) remains the reference for the rest of the pregnancy.