Understanding Pregnancy Weeks (SSW)
"How many weeks are you?" — this question accompanies every pregnancy. Yet counting pregnancy weeks (in German, SSW) often causes confusion, because it does not begin at conception. Once you understand the system, you can read any entry in your maternity record with ease.
You can see your current week any time in the <a href="/en/due-date-calculator">due date calculator</a>: it shows you the week and day, the trimester and the progress of your pregnancy.
Why counting starts from the last period
The medical week count begins on the first day of the last menstrual period — that is, roughly two weeks before conception actually takes place. This may seem illogical at first, but it has a practical reason: the exact moment of ovulation is rarely known, whereas the start of the last period almost always is. That day therefore forms the uniform starting point.
The consequence: at the moment of conception you are already in the 3rd week by this count, even though implantation is only just beginning. And the entire pregnancy lasts 40 weeks by this count, while the child's actual development spans about 38 weeks.
The "week + day" notation
In your maternity record and at the doctor's you will find figures like "12+3". This notation means: 12 completed pregnancy weeks and 3 additional days. You are then in the 13th week. The important distinction is between the completed week (12) and the current week (13) — both figures are common and refer to the same point in time.
The calculator handles this conversion for you and shows both: the current week and the exact "week+day" figure for today.
The three trimesters at a glance
Pregnancy is divided into three roughly equal periods, the trimesters. The first trimester runs until the completed 12th week (up to 12+6) and is the phase of organ formation. The second trimester covers weeks 13 to 27 (13+0 to 27+6) and is considered by many to be the most comfortable time of pregnancy. The third trimester begins with the 28th week (from 28+0) and ends with birth.
Each trimester brings its own developmental steps and check-ups. The <a href="/en/due-date-calculator">due date calculator</a> automatically assigns your current stage to the correct trimester and shows your journey through the three periods graphically.
Conclusion
Pregnancy weeks are counted from the first day of the last period, the pregnancy then lasts 40 weeks, and the "week+day" notation states completed weeks plus remaining days. With this knowledge you can read your maternity record with ease. Keep track of your current week and trimester in the <a href="/en/due-date-calculator">due date calculator</a>.