Case 1
- When did your last period start?
- 20465
- How long is your cycle?
- 28d
- Work it out for which day?
- 20671
29weeks
Third trimester
Open with these valuesPregnancy Week Calculator
0weeks
Result: 0 weeksFirst trimester
0
Gestational age is counted from the first day of your last period, not from conception. From 12 January to 6 August 2026 with a 28-day cycle that is 206 days, so 29 completed weeks — written 29+3. A longer cycle moves day zero by the difference.
Worked examples
Case 2
- When did your last period start?
- 20465
- How long is your cycle?
- 32d
- Work it out for which day?
- 20671
28weeks
Third trimester
Open with these valuesCase 3
- When did your last period start?
- 20629
- How long is your cycle?
- 28d
- Work it out for which day?
- 20671
6weeks
First trimester
Open with these valuesHow it's calculated
weeks = floor((date − day zero) / 7), day zero = LMP + cycle − 28
- StepTake the first day of your last period as day zero.
- StepMove day zero by your cycle length minus 28 days — a 32-day cycle pushes it four days later.
- StepCount the days from day zero to the date you want.
- ResultDivide by seven and drop the remainder: the whole weeks are your gestational age.
What this number means
The output is a count of completed weeks, not a due date and not a measurement taken from the pregnancy itself. It answers one question: how far the obstetric counting convention has run between two dates you supply. Both date fields open on today, so the calculator starts at zero weeks and shows something only once the first field is moved back. Take the example this page carries. A last period beginning on 12 January 2026, a cycle of 28 days, and 6 August 2026 as the day in question: day zero is the first day of that period, from there to 6 August is 206 days, and 206 divided by seven is 29 with 3 days over — 29 completed weeks, written 29+3. Set the cycle to 32 days and day zero moves four days later, to 16 January, leaving 202 days: the same day now reads 28+6. Read the figure as whole weeks only. The leftover days are dropped, so it stands still for seven days before it steps up by one, and the trimester bands beside it follow the same convention. The count is also only as good as the date it starts from: where maternity notes give a date established by ultrasound, that date is the one that applies, not the one worked out here.
Counted from your last period, not conception
Obstetrics anchors gestational age on the first day of the last menstrual period by convention, because that is the one date that can be observed. The side effect is that a pregnancy is already two weeks along on the day it begins.
What a reading like 29+3 means
29 completed weeks and 3 extra days, with the second number always between 0 and 6. This calculator shows the first number.
Your cycle length moves day zero
The plain rule assumes ovulation on cycle day 14, so a longer cycle shifts day zero by the difference. A 32-day cycle gives 28+6 where a 28-day cycle gives 29+3.
A first-trimester scan dates more precisely
Crown-rump length before 14 weeks pins gestational age to within a few days, while only about half of women recall their last period exactly. If your maternity notes give a date from a scan, that date applies rather than this calculator.
Commonly misread
At 29+3 I am in week 29, not week 30.
Both readings describe the same moment: 29 weeks completed and the thirtieth under way. Written records use completed weeks, spoken language usually names the week you are in.
Counting starts at conception.
It starts at the first day of the last period by convention, which is why a pregnancy is already two weeks along on the day it begins.
Cycle length makes no difference to the result.
A 32-day cycle puts ovulation around day 18, so the same last period gives 28+6 instead of 29+3.
Reference table
| Trimester | Completed weeks |
|---|---|
| First trimester | < 14Your value |
| Second trimester | 14 – < 28 |
| Third trimester | ≥ 28 |
Questions
What does a reading like 29+3 mean?
29 completed weeks and 3 extra days of gestational age. The first number is the full weeks behind you, the second the days past that week — always between 0 and 6. This calculator shows the first number.
Am I in week 29 or week 30 at 29+3?
Both are correct and mean the same moment: you have completed 29 weeks and are living through your thirtieth. Written records use completed weeks, spoken language usually names the week you are in.
Why does counting start at my last period, before I was pregnant?
Because the last period is the one date that can be observed. Conception cannot be pinpointed reliably, so obstetrics anchors on the first day of the last menstrual period by convention. The side effect is that a pregnancy is already two weeks along on the day it begins.
Does my cycle length change the result?
Yes — with a longer cycle you are less far along than the plain rule suggests, because it assumes ovulation on cycle day 14. A 32-day cycle puts ovulation around day 18, so the same last period gives 28+6 instead of 29+3.
When does each trimester start?
First trimester 0+0 to 13+6, second 14+0 to 27+6, third from 28+0 to birth. The boundaries follow the same completed-weeks convention as the week count itself. A birth counts as term from 37+0.
Which dating method is the most accurate?
A first-trimester ultrasound, and it is not close. Crown-rump length before 14 weeks pins gestational age to within a few days, while only about half of women recall their last period exactly. If your maternity notes give a date from a scan, trust that one over this calculator.
Sources and last check
Information, not medical advice.