Pregnancy Due Date From Conception: How Many Weeks Pregnant Are You?

Janeen Team · Sources: NICHD, MedlinePlus, CDC · accessed September 8, 2026 · 5 min read

Say you know the day you conceived: an ovulation test turned positive, or a clinic did the transfer. Add 266 days to that day and you have your estimated due date. To get the "weeks pregnant" number your clinic will use, count the weeks since conception and add two. That second step confuses almost everyone, so the rest of this page is mostly about it.

Two clocks, two weeks apart

Fetal age is time since conception. Gestational age, the number on every scan report and test schedule, starts earlier. MedlinePlus defines it as the time from the first day of the mother's last menstrual period, and spells out the oddity: during weeks 1 and 2 of pregnancy, a woman is not yet pregnant. The egg is usually released at the end of week 2. So, in MedlinePlus's words, in the 40th week of pregnancy it has been 38 weeks since conception.

That is where 266 comes from. The NICHD counts pregnancy as about 40 weeks from the last period; 40 weeks is 280 days; take away the two weeks before ovulation and 266 days remain.

Here are the three sums side by side. The arithmetic follows from the MedlinePlus and NICHD definitions above.

What you know	Do this	You get
Conception date	Add 266 days	Estimated due date
Days since conception	Divide by 7	Fetal age in weeks
Fetal age	Add 2 weeks	Gestational age (the clinic's number)

Example: conception on January 1 gives a due date around September 24. Fifty-six days after conception you are 8 weeks by fetal age and 10 weeks by the clinic's count. If a Janeen user writes in saying "the app says 10 weeks but I only conceived 8 weeks ago," this is why. Both are right; they measure different things.

Do not subtract two weeks from every clinical instruction. Screening windows, scan timing and the week-by-week milestones are all written in gestational weeks. Use the clinic's number.

How sure are you about the conception date?

Honestly, in a spontaneous pregnancy, not very. Intercourse and fertilization are not the same day, and an ovulation test marks a window rather than a moment. The NICHD notes that early signs arrive on their own vague schedule: slight bleeding in about a quarter of women 6 to 12 days after conception, breast tenderness 1 to 2 weeks after, nausea anywhere from 2 to 8 weeks after. None of these will pin the day down for you.

The exception is fertility treatment. MedlinePlus explains that in IVF the embryos are placed in the womb 3 to 5 days after egg retrieval and fertilization, so the timing is documented to the day. The CDC's assisted-reproduction glossary says that for these cycles the "last menstrual period" is simply estimated from the retrieval or transfer date. A 2000 study by Tunón and colleagues in Ultrasound in Obstetrics & Gynecology used egg retrieval plus 14 days as that stand-in and found the first-trimester ultrasound estimate differed from it by an average of just 0.9 days across 208 singleton pregnancies.

Where the ultrasound comes in

MedlinePlus lists "determine the baby's age" among the reasons for a first-trimester ultrasound, done during the first 12 weeks. The NICHD says the provider calculates your due date from the last menstrual cycle and the ultrasound exam together. In practice: you bring the dates you have, the scan measures the embryo, and if the two disagree early on, the scan usually wins.

Later scans are a different tool. They watch growth. MedlinePlus describes providers measuring the head, abdomen and thigh bone to see how well the baby is growing, not to restart the calendar. A growth scan at 30 weeks that "reads" 31 weeks is not a new due date.

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If all you have is the date you had sex

Treat it as a clue. Put the first day of your last period into a pregnancy due date calculator, bring both dates to the first visit, and let the early scan settle it. Tell the clinic if your cycles are irregular or the period date is a guess; that changes how much weight they give the scan. And whichever date is agreed, MedlinePlus is blunt about what it is: an estimate. Most pregnancies last 37 to 42 weeks.

Contact your care team the same day if you have bleeding, one-sided or severe pain, fainting, or anything that worries you. A calculator cannot tell where a pregnancy is growing or whether it is developing normally.

Frequently asked questions

How do I calculate a due date from conception?

Add 266 days to the conception date. Conception on January 1 gives an estimated due date around September 24. The figure is 280 days (the NICHD's 40 weeks from the last period) minus the two weeks before ovulation.

Why does the doctor add two weeks?

Because clinical dating starts on the first day of the last menstrual period, and MedlinePlus notes that you are not yet pregnant during weeks 1 and 2. Counting this way gives every clinic the same starting point for scans and tests.

Is intercourse date the same as conception date?

Not necessarily. It suggests a window, not the moment of fertilization. Unless the timing was documented in a fertility clinic, use menstrual history and an early ultrasound.

Is conception dating more accurate than ultrasound?

Only when the conception date is genuinely known, as after IVF. In the Tunón study of IVF pregnancies, the first-trimester scan and the known conception date agreed to within about a day on average, so for everyone else the early scan is the reliable check.

The bottom line

Conception date plus 266 days is your estimated due date, and fetal age plus two weeks is the pregnancy week your clinic will use. Use the sums for planning; use the date your care team confirms for medical decisions. Then follow along in the week-by-week guide, which counts in gestational weeks like your clinic does.

Official sources

MedlinePlus: Fetal development (gestational age, weeks 1–2, 38 weeks since conception at week 40) — accessed September 8, 2026.
NICHD: About Pregnancy (40 weeks from the last menstrual period) — accessed September 8, 2026.
NICHD: What happens during prenatal visits? — accessed September 8, 2026.
NICHD: What are some common signs of pregnancy? — accessed September 8, 2026.
MedlinePlus: Pregnancy ultrasound — accessed September 8, 2026.
MedlinePlus: Gestational age — accessed September 8, 2026.
MedlinePlus: When you pass your due date — accessed September 8, 2026.
MedlinePlus: In vitro fertilization (IVF) — accessed September 8, 2026.
CDC: ART glossary (gestational age in IVF cycles) — accessed September 8, 2026.
Tunón K et al., Ultrasound Obstet Gynecol 2000 (PubMed) — accessed September 8, 2026.

Reviewed against the official sources above. Educational information only; it is not medical advice and does not replace your maternity care team.