Pregnancy dating in 1 second — LMP, ovulation, IVF transfer (2026)

This is not medical advice. Always consult your obstetrician for pregnancy and childbirth decisions.

A positive home pregnancy test triggers a single search every time — “pregnancy week calculator”. The next questions follow in rapid succession: when is my due date, when do I book my first prenatal visit, what trimester am I in. Every one of those answers comes from a single date. That date might be the first day of your last menstrual period (LMP), the day of ovulation, or the day of an IVF embryo transfer. Which one you have determines which formula to use.

Why dating is the first thing you do

Booking the first prenatal visit, telling family or your employer, scheduling NIPT, planning maternity leave, choosing a birth setting — every one of these depends on knowing how far along you are. The first ultrasound will confirm dating, but the days between a positive test and that visit still need a working number you can plan around.

A reasonable estimate makes the rest fall into place.

The first prenatal visit at 6-10 weeks is when fetal heart tones appear.
NIPT (non-invasive prenatal screening) is most reliable from 10 weeks onward.
Pregnancy disclosure at work and FMLA paperwork start tracking from a documented week count.

If your dates are uncertain, the 6-9 week ultrasound is the gold standard for confirming or updating them, with about a 3-day margin of error. ACOG considers ultrasound dating in this window the most accurate available.

LMP, ovulation, and IVF — three different formulas

Pregnancy dating uses three standard start points, and the formula changes for each.

Start point	Days to add	Best for
LMP (last menstrual period)	+280 days	Spontaneous conception, regular 28-day cycle
Ovulation date	+266 days	OPK or basal body temperature tracking
IVF Day 3 transfer	+263 days	Cleavage-stage embryo transfer
IVF Day 5 transfer	+261 days	Blastocyst transfer (default)

LMP-based dating (Naegele’s rule) — the standard used worldwide. First day of last menstrual period plus 280 days equals the estimated due date. The formula assumes a 28-day cycle with ovulation on day 14. Pregnancy weeks are counted from the first day of the LMP, which is why you are technically “2 weeks pregnant” on the day of conception.

Ovulation-based dating — more accurate for people who tracked ovulation with OPKs or basal body temperature charts. Because ovulation typically equals LMP plus 14 days, the formula subtracts those 14 days: 280 - 14 = 266 days.

IVF transfer-based dating — the most accurate of the three. With IVF, the date of fertilization and embryo transfer are known to the hour from the lab record. For a Day 5 blastocyst transfer (the most common protocol), add 261 days. For a Day 3 cleavage-stage transfer, add 263 days. Select the IVF radio in the tool and pick Day 3 or Day 5; the formula adjusts automatically.

Get your dates in 1 second — 4 steps

The calculator at pregnancy week calculator returns a result in under a second with no signup. The flow is four steps.

Step	Action	Time
1	Pick start point (LMP, ovulation, or IVF radio)	5 sec
2	Click one date in the calendar	5 sec
3	Read result (weeks, due date, trimester)	instant
4	Review prenatal visit schedule (NIPT, anatomy scan, GBS)	instant

The result screen shows your current week and day count (e.g. 12 weeks 3 days), estimated due date, percent through the pregnancy, current trimester (1, 2, or 3), and the next milestone (heart tones, fetal movement, anatomy scan). Screenshot it and share it with your partner or family.

Trimester-by-trimester — what changes

The 40 weeks of pregnancy are divided into three trimesters, each with its own clinical focus.

Trimester	Weeks	Key items
First	0 - 13 weeks 6 days	Morning sickness, heart tones, NIPT, first-trimester screen
Second	14 - 27 weeks 6 days	Anatomy scan (18-22 weeks), glucose screen (24-28 weeks), fetal movement
Third	28 - 40 weeks	Late-trimester ultrasound, GBS test (36 weeks), birth plan, hospital tour

The first trimester carries the highest miscarriage risk; OBs commonly advise avoiding alcohol, recreational drugs, and certain medications, and limiting strenuous activity until 12 weeks. The second trimester is typically the most physically comfortable window — many parents schedule travel and prenatal classes here. The third trimester brings swelling, heartburn, and frequent urination as the body finalizes preparation for delivery.

What US expectant parents need to know — ACOG schedule

ACOG and the CDC Maternal and Infant Health program describe a typical US prenatal care schedule.

6 - 8 weeks: First prenatal visit, ultrasound to confirm dating, bloodwork, prenatal vitamins
10 - 13 weeks: NIPT (cell-free DNA screening), nuchal translucency ultrasound
15 - 20 weeks: Maternal serum quad screen (if NIPT was not done)
18 - 22 weeks: Anatomy ultrasound — fetal anatomy survey, sex determination available
24 - 28 weeks: Glucose tolerance test (gestational diabetes screen)
28 weeks: Rh immunoglobulin (RhoGAM) for Rh-negative parents
36 weeks: Group B strep (GBS) culture
36 - 40 weeks: Weekly visits, position checks, birth plan finalization

NIPT is now offered routinely in the US to all pregnant patients regardless of age, per updated ACOG guidance. Coverage by insurance varies — out-of-pocket costs typically range from $200 to $800 depending on plan and lab.

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Frequently asked questions

I do not remember my last menstrual period date exactly. How should I calculate?

Enter your best estimate, then let the first prenatal ultrasound correct it. A 6-9 week ultrasound dates the pregnancy from the size of the embryo with about a 3-day margin of error and is considered the most accurate dating method by ACOG when LMP is uncertain. The calculator shows a plus or minus 2-week range alongside the result so you can use it as a working baseline until your provider confirms dating. If your cycles are shorter than 21 days or longer than 35 days, ultrasound dating is preferred over LMP-based estimates.

I conceived through IVF. The LMP-based result and the IVF transfer-based result do not match. Which is correct?

For IVF pregnancies, the transfer-date method is considered the most accurate. LMP-based dating assumes an average cycle, but IVF gives you a documented ovulation, fertilization, and transfer timestamp. For a Day 5 blastocyst transfer, due date equals transfer date plus 261 days. For a Day 3 cleavage-stage transfer, it is transfer date plus 263 days. Select the IVF transfer radio in the tool, choose Day 3 or Day 5, and the calculator applies the correct formula automatically.

When is the first prenatal visit usually scheduled in the United States?

The American College of Obstetricians and Gynecologists recommends scheduling the first prenatal visit between 8 and 10 weeks of pregnancy, after a positive home pregnancy test. The first visit confirms the pregnancy with ultrasound, establishes a due date, reviews medical and family history, orders standard bloodwork, and begins the prenatal vitamin and folate routine. Some practices see new patients as early as 6-7 weeks if there is a history of miscarriage, ectopic pregnancy, or fertility treatment.

How accurate is Naegele's rule for setting the due date?

Naegele's rule (LMP plus 280 days) gives the estimated due date but is just an estimate — only about 4 to 5 percent of babies are born on the exact predicted date. Most births occur within plus or minus 2 weeks of the estimate, which is why obstetricians describe a 37-42 week range for full-term delivery. The rule assumes a 28-day cycle with ovulation on day 14, so it is less accurate for irregular cycles. The 6-9 week ultrasound is used to confirm or update the LMP-based date when there is a meaningful discrepancy.

How are pregnancy weeks counted? Why does my doctor say I am 8 weeks when I conceived only 6 weeks ago?

Pregnancy weeks are counted from the first day of the last menstrual period (LMP), not from the day of conception. Because ovulation typically occurs around day 14 of a 28-day cycle, you are already considered "2 weeks pregnant" on the day of conception itself. So if you conceived 6 weeks ago, your provider counts that as 8 weeks of pregnancy. The calculator follows this standard convention used by ACOG and the WHO.

Are my menstrual or transfer dates sent to a server?

No. The tool runs entirely in your browser. The values you enter never leave your device. Open the developer tools (F12) and watch the network tab — you will see zero outbound requests when you submit a date. The tool is designed this way precisely because pregnancy and cycle data are sensitive personal health information.