Reading the timeline
Pregnancy weeks and days: how gestational age is written
Pregnancy apps, scan reports, and appointments can describe the same day in ways that sound one week apart. Usually the arithmetic is fine; the language is loose.
Checked against authoritative sources
Evidence reviewed 6 August 2026 · 8 minute read
General education, not individual medical advice.
The short answer
8 weeks + 3 days means eight complete weeks and three days. Someone may casually call that “in week nine,” but clinical notation writes the completed weeks first.
What 8+3 means
The first number is the number of completed gestational weeks. The second is the number of additional days, from zero through six. After 8+6 comes 9+0.
Writing both numbers avoids the ambiguity of “week nine,” which can mean nine complete weeks to one person and the ninth week in progress to another.
Sources for this section: American College of Obstetricians and Gynecologists.
Gestational age begins at LMP by convention
Day 0 is conventionally the first day of the LMP for a non-ART pregnancy. This places the usual estimated conception around gestational week two in a classic 28-day cycle.
It is a dating framework, not a claim about the biological start of pregnancy.
Sources for this section: American College of Obstetricians and Gynecologists.
Calculating weeks from an established EDD
An established EDD represents 40+0. Count the calendar days between the target date and the EDD, subtract from 280, then divide by seven into complete weeks and remaining days.
Date-only arithmetic matters. Calculating with clock hours across daylight-saving changes can create an off-by-one error that has nothing to do with gestation.
Sources for this section: American College of Obstetricians and Gynecologists.
Where trimester boundaries fall
Using ACOG's consumer definitions, the first trimester runs through 13+6, the second from 14+0 through 27+6, and the third begins at 28+0.
Different resources sometimes group weeks differently. Name the convention instead of presenting a boundary as universal.
Sources for this section: American College of Obstetricians and Gynecologists.
Term labels use exact week boundaries too
Early term begins at 37+0, full term at 39+0, late term at 41+0, and postterm at 42+0 under ACOG definitions.
These categories are clinical terminology, not a recommendation to make a delivery decision without individualized care.
Sources for this section: American College of Obstetricians and Gynecologists.
Use the date documented by your care team
A self-entered LMP, an app estimate, and a clinician-established EDD can produce different week counts. Once clinical dating is established, use that EDD consistently in trackers and conversations.
If the displayed week appears wrong, first check the saved due date and time zone. Ask the maternity care team before changing the clinical date.
Sources for this section: American College of Obstetricians and Gynecologists.
Check the notation at important boundaries
Week labels change at midnight in the local calendar: 13+6 becomes 14+0, 27+6 becomes 28+0, and 38+6 becomes 39+0. A tracker should make that transition from a date-only due-date calculation, not from elapsed hours since a timestamp.
These boundaries have names in clinical guidance, but the label alone does not decide care. For example, 39+0 begins ACOG's full-term category; it does not create a universal recommendation about when an individual pregnancy should deliver.
When sharing a week count, include the date it applies to. “Currently 18+2 on 6 August” is auditable; a screenshot saying only “week 19” becomes ambiguous a few days later. This is especially useful when partners, apps, and care teams use different shorthand.
- 13+6: last day of the first trimester under ACOG's convention.
- 14+0: first day of the second trimester.
- 27+6: last day of the second trimester.
- 28+0: first day of the third trimester.
- 38+6: last day of early term.
- 39+0: first day of full term.
Sources for this section: American College of Obstetricians and Gynecologists, American College of Obstetricians and Gynecologists.
Common questions
Am I nine weeks pregnant at 8+3?
You have completed eight weeks and are in the ninth week. For clarity, say 8 weeks + 3 days.
When does the second trimester start?
ACOG's convention starts the second trimester at 14+0.
Why is my app one day different?
Time-zone or clock-based date arithmetic can cause an off-by-one result. Apps should use local calendar dates, and you should confirm the stored EDD.
Sources and review notes
We favour public-health guidance, clinical bodies, official statistics, and original research. Source links are provided so you can inspect the wording and boundaries yourself.
- 01Methods for Estimating the Due DateAmerican College of Obstetricians and Gynecologists · May 2017; reaffirmed 2025
- 02How Your Fetus Grows During PregnancyAmerican College of Obstetricians and Gynecologists
- 03Pregnancy due date calculatorNHS
- 04Obstetric data definitions: term pregnancyAmerican College of Obstetricians and Gynecologists