Why is pregnancy counted from your last period?

Because the first day of your last period is a date you can actually name — the day of conception almost never is. So the count starts there, about two weeks before the egg is fertilized, and every “weeks pregnant” number runs about two weeks ahead of the embryo’s real age. Below: how to put your own dates, test readouts and scan results on that one clock.

Written by Mira Chen, founder and editorPublished 2026-09-29Updated 2026-09-29

Week 1

is your period

≈ 2 wks

from period to conception*

40 wks

= 280 days from the period

38 wks

= 266 days from conception

*On a textbook 28-day cycle. Sources: ACOG Committee Opinion 700; Merck Manual.

Why the clock starts before conception

Conception happens with no signal you can see. The start of a period is something you notice and can put on a calendar. Pregnancy dating standardized on the date people can report, and ACOG describes the 280-day rule as exactly that — a convention built on a 28-day cycle with ovulation on day 14.

You can name the date

Period day 1 is visible. The day an egg was fertilized is not — even people tracking ovulation usually know a window, not a day.

One ruler for everyone

Midwives, scan reports, due-date wheels and apps all count from the last period, so “12 weeks” means the same thing everywhere.

Then it gets checked

Only about half of people recall that date accurately (ACOG) — one reason an early ultrasound confirms or corrects it.

Two clocks, one pregnancy

Schematic, drawn for a textbook 28-day cycle with ovulation on day 14 — the assumption behind the 280-day rule (ACOG). Later ovulation widens the gap; earlier ovulation narrows it.

Schematic for a 28-day cycle with ovulation on day 14 (ACOG). Later ovulation widens the gap.

Read the top line as the calendar your midwife uses and the bottom line as the embryo’s actual age. They run side by side, about two weeks apart, for the whole pregnancy. If your periods are irregular, the real gap can be more or less than two weeks (Merck Manual).

“6 weeks pregnant” means

6 weeks since the first day of your last period
About 4 weeks since conception, on a 28-day cycle
The same clock your scan report and due-date calculator use

It does not mean

You conceived 6 weeks ago
You were pregnant during that period — weeks 1–2 come before conception
Something is wrong because it “feels too early” to be 6 weeks

Translate the date you know into pregnancy weeks

Whatever you know — a period date, a conception date, an IVF transfer or a home-test readout — it converts onto the same clock. The table follows one demo pregnancy through each route. Every route gives the same week and the same due date.

Demo dates for one pregnancy, checked independently. Rules: period and conception dates (Merck Manual); IVF transfers (ACOG Committee Opinion 700).
What you know	Demo date	Rule	Weeks on Sep 14, 2026	Due date
First day of last period	Aug 3, 2026	Count from that day	6+0	May 10, 2027 + 280 days
Conception date	Aug 17, 2026	Days since conception + 14	6+0 (4 weeks since conception)	May 10, 2027 + 266 days
Day-5 embryo transfer	Aug 22, 2026	Transfer day counts as 2+5	6+0	May 10, 2027 + 261 days
Day-3 embryo transfer	Aug 20, 2026	Transfer day counts as 2+3	6+0	May 10, 2027 + 263 days

Home tests with a weeks indicator

These tests estimate weeks since conception from hCG levels. The maker’s leaflet says your doctor will date the pregnancy from your last period instead (Clearblue).
Test shows	Meaning on the test	Pregnancy weeks (28-day cycle)
1–2	Conceived about 1–2 weeks ago	About 3–4 weeks
2–3	Conceived about 2–3 weeks ago	About 4–5 weeks
3+	Conceived more than 3 weeks ago	5 weeks or more

Weeks pregnantdays since last period ÷ 7 · remainder = extra days

From a conception datedays since conception + 14

From a weeks-indicator testreadout + about 2 weeks

Due datelast period + 280 = conception + 266

For your own dates, the due date calculator runs each of these routes — your period date with your real cycle length, a conception date, or a day-3 or day-5 transfer — and shows today’s week and your due date.

Reading “6+3”: which week are you in?

Pregnancy weeks are counted as completed weeks, like age in years. Scan reports and apps write the same thing three ways — 6+3, 6w3d or 6 3/7 — all meaning 6 full weeks and 3 days.

6+0

day 42 · first day of week 7

6+3

day 45 · still in week 7

13+6

day 97 · last day of the first trimester

Completed-week notation as used in ACOG guidance (for example “13 6/7 weeks”); day counts are simple arithmetic.

So “I’m 6 weeks” and “I’m in week 7” describe the same day. An app that says “week 7” is not ahead of your midwife.

If your cycle isn’t 28 days

The two-week gap assumes ovulation on day 14. In a longer cycle, ovulation usually comes later, so a plain count from the period runs ahead of the pregnancy; in a shorter cycle it runs behind. For regular cycles the standard correction is one day per day of difference: due date = last period + 280 + (cycle length − 28) (Merck Manual).

Assumes ovulation 14 days before the next period. Real luteal phases run about 10–15 days (Bull et al., npj Digital Medicine 2019), so read each row as ± a few days.
Your usual cycle	Ovulation around	A plain count from the period is…	Due date moves
24 days	Day 10	4 days behind	4 days earlier
26 days	Day 12	2 days behind	2 days earlier
28 days	Day 14	Right on	No change
30 days	Day 16	2 days ahead	2 days later
32 days	Day 18	4 days ahead	4 days later
35 days	Day 21	7 days ahead	7 days later
40 days	Day 26	12 days ahead	12 days later

Demo, same period date (Aug 3, 2026), 35-day cycle. Ovulation lands around day 21 instead of day 14. On Sep 14 a plain count says 6+0; the cycle-adjusted count says 5+0, and the due date moves from May 10 to May 17, 2027.

With irregular cycles there is no reliable day to correct from. In that case the Merck Manual notes that an early ultrasound may be the only source of the due date.

When the scan gives a different week

A first-trimester scan measures the embryo from crown to rump. Up to 13+6 weeks it is the most accurate way to set the pregnancy’s age — to within about ±5–7 days (ACOG). When it disagrees with the count from your period, the usual reasons are ordinary:

Ovulation came later or earlier than day 14. Common with long or variable cycles — the embryo is simply younger or older than the calendar assumed.
The period date was off. About half of people recall it accurately (ACOG), and spotting mistaken for a period shifts it too.
Every scan has a margin. Small differences sit inside it, which is why they are ignored.

When the due date is changed (US guidance)

ACOG Committee Opinion 700 thresholds, as tabulated in the Merck Manual Professional. Dates are reconciled once, at the first accurate scan — later scans do not move the due date. Other countries’ pathways differ; in England the dating scan is offered at around 10–14 weeks (NHS).
Pregnancy week at the first scan	Scan dating is used if it differs from the period count by more than
Up to 8+6	5 days
9+0 to 15+6	7 days
16+0 to 21+6	10 days
22+0 to 27+6	14 days
28+0 or later	21 days

Demo: period started Jul 6, 2026
At a scan on Aug 24, the count from the period says 7+0.
The embryo measures like 5+6
That fits an ovulation about 8 days later than day 14.
8 days is more than the 5-day limit before 9 weeks
Under the ACOG table, the scan dating would be adopted: due date Apr 20, 2027 instead of Apr 12.

What this page cannot tell you. A scan that measures smaller than expected is often timing, but not always — sometimes a follow-up scan is needed to know. Only the team that did the scan can say which applies to you. The warning signs that need urgent care in early pregnancy are listed in the positive-test section of the missed period guide.

Weeks, months and trimesters

Forty weeks is 280 days — about 9.2 average calendar months, so “nine months” is a rounded figure. Weeks are more precise than months, which run anywhere from 28 to 31 days.

Demo pregnancy with a period date of Aug 3, 2026 (due May 10, 2027). Trimesters: Merck Manual; term labels: ACOG Committee Opinion 579. The NHS gives 37–42 weeks from the last period as the normal length of pregnancy.
Stage	Pregnancy weeks	Demo dates
First trimester	Up to 13+6	Until Nov 8, 2026
Second trimester	14+0 to 27+6	Nov 9, 2026 – Feb 14, 2027
Third trimester	28+0 to birth	From Feb 15, 2027
Early term	37+0 to 38+6	Apr 19 – May 2, 2027
Full term	39+0 to 40+6	May 3 – May 16, 2027 · due date May 10 = 40+0
Late term	41+0 to 41+6	May 17 – May 23, 2027
Postterm	42+0 onward	From May 24, 2027

How often babies arrive on the due date itself — and why it is a planning anchor, not a deadline — is covered with the due date calculator.

Which number to use when

Booking or talking to your midwifeWeeks from your last period

It is the clinical clock.

After your first accurate scanThe scan’s dates

They replace the period estimate when the gap passes the threshold above.

A weeks-indicator test resultReadout + about 2 weeks

The test counts from conception.

An IVF pregnancyTransfer-based dates

The embryo’s age is known exactly (ACOG).

Reading week-by-week articlesCheck which clock it uses

Some embryology sources count from fertilization — two weeks lower.

What would you like to do next?

Your own week and due date

From your period date and cycle length, a conception date, or an IVF transfer.

Due date calculator →

When did conception happen?

Back-calculate the likely conception window from your dates.

Conception calculator →

Not tested yet?

When a result becomes reliable after a missed period.

Pregnancy test timing →

Frequently asked questions

Am I pregnant during weeks 1 and 2?

No. Week 1 is your period and week 2 is the egg maturing; conception happens around the end of week 2 on a 28-day cycle. The weeks are counted anyway because the pregnancy is dated from the first day of that last period.

If I conceived 3 weeks ago, how many weeks pregnant am I?

About 5 weeks. Add two weeks to the time since conception: 3 + 2 = 5. The due date calculator does this from a conception date and also gives the due date (conception + 266 days).

Why does my test say 2–3 weeks but my doctor says 5?

Weeks-indicator tests estimate time since conception; doctors count from your last period, about two weeks earlier. A “2–3” readout corresponds to roughly 4–5 weeks pregnant on a 28-day cycle (Clearblue).

Is pregnancy 40 weeks or 9 months?

Both, roughly. 40 weeks is 280 days — about 9.2 average calendar months. Clinicians use weeks because months vary in length.

Does a long cycle change how far along I am?

It can. With a regular 35-day cycle, ovulation is around day 21, so a plain count from your period runs about a week ahead; the due date moves about 7 days later (Merck Manual formula: last period + 280 + cycle length − 28).

My scan says I’m less far along than my period date. Is something wrong?

Often it is timing — later ovulation or an uncertain period date — but a scan alone can’t always tell. If the gap is more than the ACOG threshold for that stage (5 days before 9 weeks), the scan dates replace the period estimate. Your scan team is the one to interpret it.

Will a later scan change my due date?

Normally not. Under ACOG guidance the dates are reconciled once, at the first accurate ultrasound; later scans are less precise for dating and are not used to move the due date.

How are pregnancy weeks counted after IVF?

From the transfer, using the embryo’s known age: a day-5 embryo transfer day counts as 2 weeks 5 days, a day-3 transfer as 2 weeks 3 days. Due date = transfer + 261 days (day 5) or + 263 days (day 3), per ACOG.

References

This tool provides estimates for personal planning only and is not a substitute for professional medical advice, diagnosis or treatment. Predictions must not be used as contraception. If you have concerns about your cycle, heavy bleeding, severe pain or missed periods, please consult a licensed clinician.