Pregnancy dating guide
Pregnancy dates and weeks: what the numbers mean
Pregnancy dates sound exact because they are written as a day on a calendar. Clinically, they are an agreed estimate used to organize care, while the actual birth date remains uncertain.
The short answer
The conventional estimated due date is 280 days from the first day of the last menstrual period. Early ultrasound can establish or confirm dating more accurately, and IVF pregnancies use the embryo age and transfer date.
ACOG convention
Three trimesters, written in weeks plus days
The boundaries are exact; symptoms and development do not change on a single midnight.
Dating methods
The starting information determines the calculation
Last menstrual period
- Start with first day of LMP
- Add 280 calendar days
- Adjust only for a reliable cycle length
IVF transfer
- Use the clinic transfer date
- Day-5 embryo: add 261 days
- Day-3 embryo: add 263 days
Once the care team establishes an EDD, use that documented date consistently.
Explore this guide
Every article in this guide.
9 minute read
How pregnancy due dates are calculated
See the 280-day LMP convention, cycle-length adjustment, and IVF formulas, with the assumptions and clinical limits attached.
8 minute read
Pregnancy weeks and days: how gestational age is written
Understand gestational age notation such as 8+3, why “week nine” can be ambiguous, and how weeks are calculated from an established due date.
8 minute read
Pregnancy trimesters explained with exact week boundaries
See ACOG trimester boundaries in weeks plus days, understand why sources differ, and avoid treating calendar months as exact clinical units.
9 minute read
Ultrasound and due dates: when dating is most accurate
Learn how first-trimester ultrasound dates a pregnancy, what crown–rump length establishes, and why later scans rarely change the due date.
8 minute read
IVF due-date calculation from embryo transfer
Understand ACOG's IVF dating convention for day-3 and day-5 embryo transfer, why ART dating takes priority, and how to read the result.
Free tool
Pregnancy due-date calculator
Estimate a due date from LMP, cycle length, or IVF transfer, with the exact convention shown beside the result.
A due date is a clinical estimate
The estimated due date, or EDD, gives prenatal care a shared timeline. It helps clinicians interpret growth, schedule time-sensitive care, and describe gestational age consistently.
It is not a promise of the birth date. ACOG describes the average pregnancy as 280 days, or 40 weeks, counted from the first day of the last menstrual period. The NHS notes pregnancy commonly lasts 37 to 42 weeks from that same starting point.
Sources for this section: American College of Obstetricians and Gynecologists, NHS.
Why counting starts before conception
Gestational age conventionally begins on day one of the last menstrual period, even though conception usually occurs later. This provides an observable date in a process where ovulation and fertilization are often not known exactly.
The classic formula assumes a 28-day cycle with ovulation around day 14. Irregular cycles, uncertain recall, unusual bleeding, or later ovulation can make an LMP estimate less reliable. Gestational dating is a convention; it does not mean a person was biologically pregnant on the first day of that period.
Sources for this section: American College of Obstetricians and Gynecologists.
Use the best source available for the pregnancy
ACOG says first-trimester ultrasound, through 13 weeks and 6 days, is the most accurate method to establish or confirm gestational age. Crown–rump length dating in this period has an accuracy of about plus or minus five to seven days.
For pregnancies resulting from assisted reproductive technology, the ART-derived age should be used. Once LMP and the first accurate ultrasound are available, the EDD should be discussed and documented; later changes should be rare and clinician-led.
- Known IVF transfer and embryo age: use ART-derived dating.
- Reliable LMP plus early ultrasound: the clinician reconciles them using established guidance.
- Uncertain LMP or irregular cycles: a calendar calculator should not overrule clinical dating.
Sources for this section: American College of Obstetricians and Gynecologists.
Weeks plus days is the clearest notation
A gestational age such as 8 weeks + 3 days means eight complete weeks and three additional days. In everyday speech, somebody may call this “week nine,” but the clinical notation is less ambiguous.
Trimester boundaries use that same week-plus-day system. ACOG describes the first trimester through 13+6, the second from 14+0 through 27+6, and the third from 28+0 onward.
Sources for this section: American College of Obstetricians and Gynecologists.
Thirty-seven weeks is not the start of full term
Current obstetric terminology separates early term, full term, late term, and postterm. ACOG defines early term as 37+0 through 38+6, full term as 39+0 through 40+6, late term as 41+0 through 41+6, and postterm as 42+0 and beyond.
These labels describe gestational periods; they are not instructions about an individual delivery. Clinical decisions depend on the pregnancy and belong with the maternity care team.
Sources for this section: American College of Obstetricians and Gynecologists, American College of Obstetricians and Gynecologists.
What a calculator is good for
A calculator can apply a stated formula consistently, show milestone dates, and translate an established EDD into weeks plus days. It cannot examine the pregnancy or decide whether the input date was reliable.
Use the result to prepare for a care conversation or orient yourself before the first appointment. Once your midwife or doctor establishes an EDD, use that clinical date rather than repeatedly redating the pregnancy online.
Contact your maternity care provider if your last period is uncertain, cycles are irregular, or you are unsure which dating method applies. Do not use this tool to change an established clinical due date.
Sources for this section: American College of Obstetricians and Gynecologists, NHS.
Questions
Common questions.
- Why is pregnancy counted from the last period?
- Because the first day of the last menstrual period is often observable while conception is not. The convention assumes a 28-day cycle and ovulation around day 14.
- Can an ultrasound change my due date?
- An early ultrasound can establish or confirm gestational age. Whether to revise an EDD is a clinical decision based on timing, discrepancy, and the reliability of the LMP.
- Is 37 weeks full term?
- ACOG defines 37+0 through 38+6 as early term. Full term begins at 39+0 and runs through 40+6.
- Should I use LMP dating after IVF?
- No. ACOG recommends ART-derived dating using embryo age and transfer date for pregnancies resulting from IVF.
Sources
Sources and review notes.
- 01 American College of Obstetricians and Gynecologists
- Methods for Estimating the Due DateMay 2017; reaffirmed 2025
- 03 American College of Obstetricians and Gynecologists
- How Your Fetus Grows During Pregnancy
- 04 American College of Obstetricians and Gynecologists
- When Pregnancy Goes Past Your Due Date
- 05 American College of Obstetricians and Gynecologists
- Obstetric data definitions: term pregnancy