Start With the Gestational Clock, Not the Conception Clock
Pregnancy is conventionally dated from the first day of the last menstrual period, even though fertilization usually occurs later. In a model 28-day cycle, ovulation and conception are placed around day 14. A pregnancy described as 10 weeks gestational age is therefore modeled as about eight weeks from conception. The first two gestational weeks are part of a standardized timeline, not a claim that pregnancy began before fertilization.
Choose the Strongest Available Dating Basis
An LMP estimate is practical when the date is certain and cycles are regular. It becomes weaker when bleeding was unusual, cycles vary, ovulation timing is unpredictable, or the person does not remember the first day accurately. A cycle-length adjustment can shift an estimate, but it cannot observe ovulation.
A known conception or ovulation date can establish a useful timeline when supported by fertility tracking or treatment records. For assisted reproductive technology, ACOG says ART-derived gestational age should assign the EDD. For spontaneous pregnancy, ACOG identifies first-trimester ultrasound measurement as the most accurate method to establish or confirm gestational age.
Follow Each Due Date Formula
Every method is converted to the same 280-day gestational scale. LMP mode begins at the reported menstrual date and shifts by the cycle-length difference from 28 days. Known conception begins 14 days after the LMP-equivalent start. Ultrasound mode works backward from the report's gestational age on the scan date.
IVF dating uses embryo age because a day-5 embryo has already developed two days longer than a day-3 embryo at transfer. ACOG's examples place a day-5 transfer 261 days before the EDD and a day-3 transfer 263 days before the EDD.
Formula notes
LMP EDD = LMP + 280 days + (cycle length - 28 days).Conception EDD = known conception or ovulation date + 266 days.Ultrasound EDD = scan date + 280 days - report gestational age in days.Day-5 transfer EDD = transfer date + 261 days.Day-3 transfer EDD = transfer date + 263 days.Gestational age on a date = selected date - LMP-equivalent gestational start.
Read Weeks and Days Without Forcing Them Into Months
Gestational age is written as completed weeks plus additional days. The notation 14 weeks 1 day means 99 days have elapsed from the gestational start and week 15 is underway. The day count resets after six because seven additional days complete another week.
Pregnancy months are imprecise. Calendar months range from 28 to 31 days, while four weeks always equal 28 days. Ten groups of four weeks equal 40 weeks but only 280 days, which is shorter than ten average calendar months. Weeks and days are therefore the clearer language for records and appointments.
- First trimester: gestational start through 13 weeks 6 days.
- Second trimester: 14 weeks 0 days through 27 weeks 6 days.
- Third trimester: begins at 28 weeks 0 days.
- The selected calculate-on date changes gestational age but not the established EDD.
Compare Five Inputs That Reach One Timeline
Consider an LMP of April 1, 2026 and a usual 28-day cycle. Adding 280 days gives January 6, 2027. A known conception date of April 15 reaches the same EDD after 266 days. A day-5 embryo transferred April 20 also reaches January 6 after 261 days.
An ultrasound report stating 14 weeks 1 day on July 9, 2026 reaches the same gestational start because 99 days are subtracted from the scan date. On July 9, each aligned method reports 14 weeks 1 day and the second trimester. Agreement is possible because each input describes a different point on the same gestational scale.
Keep the Estimated Due Date in Perspective
An EDD is a reference for gestational age, growth tracking, and care planning; it is not a promise about the day of birth. ACOG's patient guidance says only about one in 20 women gives birth on the due date. A calendar result should therefore be read as a dating anchor rather than a countdown to an exact event.
Once the care team selects an EDD, it is not normally changed because each later ultrasound produces a different size estimate. ACOG explains that decisions to revise dating affect pregnancy management and should be discussed and recorded. Keep the established date from the medical record even when a consumer calculator produces a nearby alternative.
Understand Term Labels as Timing Categories
ACOG and the Society for Maternal-Fetal Medicine classify 37w0d-38w6d as early term, 39w0d-40w6d as full term, 41w0d-41w6d as late term, and 42w0d onward as postterm. These are timing labels, not instructions to choose a delivery date or delay an evaluation; individual decisions remain with the care team.
Use the Result for Orientation, Then Use Clinical Care
Check that the source date, cycle length, ultrasound age, or embryo age came from the correct record. Confirm that the calculate-on date is the day you intend to inspect. If the result differs from the EDD in the chart, do not average the dates; ask the care team which basis was used and keep the documented EDD for care planning.
A date calculator cannot confirm pregnancy, diagnose pain or bleeding, assess fetal development, interpret an ultrasound image, or determine whether symptoms are urgent. CDC advises starting prenatal care early and seeking immediate medical care for urgent maternal warning signs. If something does not feel right, contact a healthcare professional and state that you are pregnant or recently pregnant.
- Verify the first day of LMP rather than the final day of bleeding.
- Treat cycle length as an estimate unless ovulation timing is known.
- Copy ultrasound weeks and days from the report exactly.
- Confirm whether the transferred embryo was day 3 or day 5.
- Use the clinician-established EDD for medical decisions and records.
- Seek immediate care for urgent symptoms rather than consulting a calendar result.
Frequently asked questions
How many weeks pregnant am I from my last period?
Choose LMP and enter the first day of the last menstrual period. The calculator counts completed weeks and additional days from the cycle-adjusted gestational start to the selected date. If cycles are irregular or the LMP is uncertain, compare the result with the EDD established by a clinician.
How is a pregnancy due date calculated from LMP?
The conventional estimate is 280 days from the first day of the LMP. This page also adds the difference between the usual cycle length and 28 days. That adjustment is a model of ovulation timing, not proof of the actual conception date.
Why does pregnancy counting start before conception?
Gestational age traditionally begins on the first day of the LMP. In a typical 28-day cycle, conception is modeled about 14 days later, so the gestational clock includes roughly two weeks before conception.
Can I use this pregnancy calculator with irregular periods?
You can enter a usual cycle length from 20 to 45 days, but cycle adjustment cannot model unpredictable ovulation or uncertain bleeding. ACOG states that first-trimester ultrasound is the most accurate method to establish or confirm gestational age, especially when LMP is unreliable.
References
These sources support the method or guidance used for Pregnancy Calculator. Verify time-sensitive rules at the source.
Try the calculator
Open Pregnancy Calculator, enter your scenario, and compare its supporting rows with this guide's method and checks.