Pregnancy Week Calculator
Enter the first day of your last period – see your current week instantly
- Updated
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Default 28 days. Longer cycles move the due date later – as in the due date calculator.
Calculated for today, .
Week of pregnancy
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–Please enter a cycle length between 21 and 45 days.
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 provide three ultrasound scans in fixed windows: 8+0 to 11+6 (weeks 9–12), 18+0 to 21+6 (weeks 19–22) and 28+0 to 31+6 (weeks 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.
The three trimesters at a glance
The calculator counts trimesters as defined by familienplanung.de (German Federal Institute of Public Health, glossary entry “Trimenon / Trimester”): 1st trimester weeks 1–13 (up to 12+6), 2nd trimester weeks 14–26 (13+0 to 25+6), 3rd trimester from week 27 (from 26+0). Some sources count differently, e.g. the 3rd trimester only from week 28.
Developmental phases by week
- 1st trimester: weeks 1–13 (up to 12+6)
- Embryonic phase: implantation, organ development, heartbeat from week 6. Miscarriage risk is highest. First-trimester screening around week 12.
- 2nd trimester: weeks 14–26 (13+0 to 25+6)
- Fetal phase: visible growth, first fetal movements (around week 18–22). Anomaly scan around week 20. Miscarriage risk drops sharply.
- 3rd trimester: from week 27 (from 26+0)
- Maturation phase: weight gain, lung development, birth preparation. Birth from week 37 counts as full-term. Due date at week 40+0.
- Post-term: from week 42
- From week 42+0 the pregnancy is considered post-term. Closer monitoring is recommended; induction depends on the clinical findings.
Calculation examples
- LMP
- Jan 1, 2026
- Reference date
- Jan 22, 2026
- Days since LMP
- 21 days
- Week = floor(21/7)
- 3+0
- LMP
- Jan 1, 2026
- Reference date
- Mar 23, 2026
- Days since LMP
- 81 days
- Week = floor(81/7)
- 11+4
Frequently asked questions about the pregnancy week calculator
Counting method, meaning and developmental phases
How is the pregnancy week (gestational age) calculated?
The pregnancy week is counted from the first day of the last menstrual period (LMP). The number of days since the LMP is divided by 7 and rounded down: week = floor(days / 7). The remainder forms the second part in the format "week+remaining days" (e.g. 12+3 for 12 completed weeks, day 3). On the LMP day itself, week 0+0 begins; the estimated due date (280 days later) falls on week 40+0.
What does the format "week X+Y" mean?
The format "week X+Y" means: X is the completed pregnancy week, Y the day within the current week (0 to 6). "12+3" therefore means: week 12 is complete, and you are on the third day of week 13. In medicine, the phrasing "week 12 + 3 days" is also common.
Why does the week count start with the last period?
The week count starts with the first day of the last menstrual period because this point in time is easy to determine. The actual moment of conception is typically about 14 days later (with a 28-day cycle). Medically, the pregnancy therefore begins from week 1, even though conception itself happens around week 3.
From which week is a pregnancy considered safe?
The risk of miscarriage drops significantly after week 12, because the critical phase of organ development is complete. A premature baby is considered "viable" from around week 22–24 (with intensive care support). In Germany, a birth from week 37 onward is classified as full-term.
What is the difference between pregnancy week and gestational age?
Pregnancy week and gestational age are synonymous and are both counted from the first day of the last period. The term "fetal age" refers to the time since conception (roughly 2 weeks less than the pregnancy week).