Pregnancy is counted from the last period
Every pregnancy calculator, ultrasound report and maternity record uses the same starting point: the first day of the last menstrual period. It may seem odd to start counting before conception, but there is a practical reason. Most people know — or can look up — when their last period started, while the day of ovulation and conception happens unnoticed and is rarely known for certain. A shared, knowable starting point means that “12 weeks” means the same thing to you, your midwife and the sonographer.
In a typical 28-day cycle, ovulation happens around day 14. So by the time conception takes place, the pregnancy is already counted as two weeks along. That is why a positive home test at the time of a missed period corresponds to about 4 weeks pregnant, even though conception was only about two weeks earlier.
Gestational age and fetal age
The age measured from the last period is called gestational age. The baby’s actual age since conception is the fetal age (or conceptional age), and it is about two weeks less. At 20 weeks pregnant, for example, the baby has been developing for roughly 18 weeks. Week-by-week guides, including ours, describe gestational age unless they say otherwise.
| Gestational age | Approximate fetal age |
|---|---|
| 4 weeks | 2 weeks |
| 12 weeks | 10 weeks |
| 20 weeks | 18 weeks |
| 40 weeks | 38 weeks |
Reading “weeks and days”
Gestational age is written as completed weeks plus days: 20+3, 20w3d, or “20 weeks and 3 days”. It works like a person’s age in years — the number only goes up when a full week is completed. On the first day of the last period you are 0+0; seven days later you are 1+0. While you are 20+0 to 20+6, you are in the 21st week of pregnancy. The two ways of speaking (“20 weeks pregnant” and “in week 21”) describe the same days, which is a common source of confusion.
To work it out yourself, count the days from the first day of your last period to today and divide by seven. The whole number is the completed weeks; the remainder is the days. If it has been 143 days, you are 20 weeks 3 days. The Pregnancy Week Calculator does this instantly and handles month lengths and leap years.
Adjusting for cycle length
The 14-day assumption fits a 28-day cycle. In longer cycles ovulation usually comes later, and in shorter cycles earlier, because the phase after ovulation tends to be fairly constant at around 14 days. If your cycles are regularly 33 days, for example, ovulation probably happened around day 19, so your pregnancy is likely about five days less advanced than a standard LMP count suggests. Calculators that let you enter a cycle length — like the Pregnancy Calculator — shift the dates accordingly.
How weeks relate to months and trimesters
A 40-week pregnancy is 280 days, a little over nine calendar months. Because months have 28 to 31 days, weeks never line up perfectly with months, and sources group them slightly differently. This site uses the common grouping:
| Pregnancy month | Weeks pregnant (completed) | Trimester |
|---|---|---|
| Month 1 | Weeks 1–4 | First |
| Month 2 | 5 to 8 weeks | First |
| Month 3 | 9 to 13 weeks | First |
| Month 4 | 14 to 17 weeks | Second |
| Month 5 | 18 to 22 weeks | Second |
| Month 6 | 23 to 27 weeks | Second |
| Month 7 | 28 to 31 weeks | Third |
| Month 8 | 32 to 35 weeks | Third |
| Month 9 | 36 to 40 weeks | Third |
Trimesters are defined directly in weeks, which makes them more precise: the first trimester runs to 13 weeks 6 days, the second from 14 weeks to 27 weeks 6 days, and the third from 28 weeks until birth.
When the dating changes
An early ultrasound measures the embryo or fetus and estimates gestational age directly. If it differs enough from your LMP-based dates, your provider may re-date the pregnancy, and from then on your weeks follow the scan. That is normal and does not mean anything is wrong — it simply means ovulation happened at a different time than the standard assumption. Read more in How Accurate Is a Due Date?