Gestational age is counted from the first day of the last menstrual period, not from conception. Conception typically occurs about two weeks after that day, so a pregnancy described as 6 weeks is about 4 weeks from fertilization. A full-term pregnancy of 40 weeks is about 38 weeks from conception.
The convention exists because the period date is usually known and the conception date usually is not. When the due date is established by ultrasound or IVF instead, the count is adjusted. It still reads as if counted from a period date 280 days before the due date.
Completed weeks and days
A gestational age of 24 weeks 3 days means that 24 full weeks have been completed and the 25th week is in progress. The due date calculator reports gestational age in this form for today or any date entered, and shows the calendar dates on which each week begins and ends.
The notation is sometimes written 24+3 or 24w3d. All three forms mean the same thing. Some sources describe a person "in their 25th week" at this point, which is correct but often misread as 25 weeks. The completed-weeks form avoids the ambiguity.
Trimesters
Committee Opinion 700 and related guidance divide pregnancy into three trimesters by completed weeks. The first trimester runs through 13 weeks 6 days, the second from 14 weeks 0 days through 27 weeks 6 days, and the third from 28 weeks 0 days. Other sources use slightly different boundaries, which is one reason two references can place the same date in different trimesters.
Weeks and months
Forty weeks is about nine months and one week, not ten months, and not nine months of four weeks each. Calendar months average about 4.35 weeks. A pregnancy at 20 weeks is at about four and a half months. At 36 weeks it is at about eight and a quarter months.
Month-based descriptions of pregnancy are imprecise for this reason, and clinical care uses weeks and days throughout.
Term
Committee Opinion 579 defines term as 37 weeks 0 days through 41 weeks 6 days. Early term runs from 37 weeks 0 days to 38 weeks 6 days. Full term runs from 39 weeks 0 days to 40 weeks 6 days, and late term from 41 weeks 0 days to 41 weeks 6 days. Post-term begins at 42 weeks 0 days. Preterm is any birth before 37 weeks 0 days. Each boundary is stated in completed weeks and days, and a birth at 36 weeks 6 days is preterm.
Why counting from the period date causes confusion
The two-week offset between the period date and conception produces the most common misunderstanding. A person who conceived on a known date and counts from it will be about two weeks behind their clinician's count. The clinician's count is the one used for every screening window, appointment schedule, and the definition of term above.
Wilcox and colleagues (1995) found that the fertile window spans about six days ending on the day of ovulation, and that conception does not usually occur on the day of intercourse. A count from a single date of intercourse is therefore also imprecise, independent of the two-week offset.
When the count is corrected
If a first-trimester ultrasound differs from the period-based date by more than the margin in Committee Opinion 700, the due date is changed and the gestational-age count changes with it. A person who was 9 weeks 2 days by period date may become 8 weeks 5 days after the scan. The calculator shows this as a comparison between candidate dates rather than making the change itself, because the decision to redate belongs to the care team.