What Is a Pregnancy Calculator
A pregnancy calculator turns one established dating reference into an estimated 40-week gestational timeline. This page can start from the first day of the last menstrual period (LMP), an already established due date, a known conception or ovulation date, gestational age written on an ultrasound report, or the date and embryo age for an IVF transfer.
Gestational age is reported in completed weeks and additional days. It conventionally begins on the first day of the LMP, about two weeks before conception in a typical 28-day cycle. That convention is why a person may be described as four weeks pregnant roughly two weeks after conception.
How to Use the Pregnancy Calculator
Choose the dating method that matches the most reliable information you have. Enter dates exactly as recorded, then select the date on which you want to know gestational age. The result identifies its dating basis so an LMP estimate cannot be confused with an ultrasound- or clinic-established EDD.
If a clinician has already established a due date, use that date rather than repeatedly recalculating from later ultrasounds. ACOG notes that changes to an established EDD affect pregnancy management and should be discussed and documented by the care team.
- Select LMP, known due date, conception, ultrasound gestational age, or IVF transfer.
- Enter the source date and any method-specific detail, such as usual cycle length or embryo age.
- Leave the calculate-on field at today or choose another date for a past or future gestational-age check.
- Calculate and read the dating basis, due date, weeks and days, trimester, term designation, and milestone table together.
- Compare the estimate with the EDD in the medical record and ask the care team about any difference.
Choose the Dating Method That Fits the Record
LMP mode is useful when the first day of the last period is known and cycles are reasonably regular. The cycle-length adjustment shifts the estimate by the difference from 28 days, but it cannot know the actual day of ovulation. Known due date mode works backward from an EDD already supplied by a clinician or fertility clinic.
Conception mode adds 266 days to a known ovulation or conception date. Ultrasound mode does not interpret an image or crown-rump length; it uses the gestational weeks and days already stated on a report. IVF mode applies the transfer date and embryo age: ACOG gives 261 days after a day-5 transfer and 263 days after a day-3 transfer.
Pregnancy Due Date Formulas
The formulas place each input on the same conventional gestational clock. A due date is an estimate, not a prediction of the day labor will begin. The calculator uses calendar-day arithmetic and does not insert an extra leap-day rule because the date library already follows the real calendar.
LMP estimate: EDD = first day of LMP + 280 days + (usual cycle length - 28 days).Known conception: EDD = conception or ovulation date + 266 days.Known EDD: estimated gestational start = EDD - 280 days.Ultrasound report: EDD = scan date + 280 days - reported gestational age in days.Day-5 IVF transfer: EDD = transfer date + 261 days.Day-3 IVF transfer: EDD = transfer date + 263 days.
How Pregnancy Weeks, Days, and Trimesters Are Counted
A result of 14 weeks and 1 day means 14 full weeks have elapsed and the pregnancy is in its fifteenth week. Clinicians normally use weeks and days because calendar months have unequal lengths and do not map cleanly to a 40-week pregnancy.
Following ACOG's patient guidance, the first trimester runs through 13 weeks and 6 days, the second runs from 14 weeks and 0 days through 27 weeks and 6 days, and the third begins at 28 weeks and 0 days. The selected date controls the displayed trimester; it does not change the estimated due date.
Early Term, Full Term, Late Term, and Postterm Dates
The timeline includes ACOG's gestational labels for deliveries at or beyond 37 weeks. Early term is 37 weeks and 0 days through 38 weeks and 6 days. Full term is 39 weeks and 0 days through 40 weeks and 6 days. Late term is 41 weeks and 0 days through 41 weeks and 6 days, and postterm begins at 42 weeks and 0 days.
These labels describe gestational timing; they are not instructions about when delivery should occur. Individual care decisions depend on the established EDD, health history, pregnancy findings, and clinician assessment.
Pregnancy Calculator Example
The same underlying timeline can be reached from several reliable inputs. These examples deliberately converge on one due date so the relationship between methods is visible.
| Dating method | Example input | Estimated due date | Important assumption |
|---|---|---|---|
| LMP | April 1, 2026; 28-day cycle | January 6, 2027 | Ovulation is modeled near cycle day 14 |
| Known conception | April 15, 2026 | January 6, 2027 | Conception or ovulation date is known |
| Ultrasound report | 14w1d on July 9, 2026 | January 6, 2027 | Weeks and days come from the report |
| Day-5 IVF transfer | April 20, 2026 | January 6, 2027 | Embryo age is five days at transfer |
| Known EDD | January 6, 2027 | January 6, 2027 | The established due date is used directly |
Reading the Pregnancy Timeline
The primary result is the estimated due date. Supporting rows show the source method, LMP-equivalent gestational start, conception estimate, gestational age on the selected date, current week underway, trimester, term designation, elapsed days, remaining days, and percentage of the conventional 280-day timeline.
The milestone table is a calendar reference, not an appointment schedule. It marks trimester boundaries and ACOG term classifications from the same dating basis. Screening, monitoring, and visit timing should come from the care team because recommendations can depend on medical history, local practice, and pregnancy findings.
Pregnancy Calculator Features
- Five dating methods with only the relevant fields shown for each method.
- Cycle-adjusted LMP calculation for usual cycles from 20 to 45 days.
- Known EDD and known conception date calculations.
- Ultrasound mode based on report-provided weeks and days, without pretending to read an image.
- Day-3 and day-5 IVF embryo-transfer dating.
- Gestational age in completed weeks and days for any selected date.
- Correct trimester boundaries and ACOG term designations.
- Milestone table, transparent formula, calculation steps, copy action, and PDF export.
- Browser-based calculation without requiring an account.
Benefits of a Method-Aware Pregnancy Calculator
Using one visible dating basis at a time reduces a common source of confusion. The result explains whether the timeline came from menstrual history, a known EDD, conception, ultrasound-reported age, or embryo transfer instead of blending dates that may carry different levels of reliability.
The shared timeline also makes the relationships auditable. Weeks and days, trimester boundaries, conception estimate, term dates, and the 40-week EDD are all derived from the same gestational start, while the clinical notice keeps a planning calculation separate from individual care.
Common Pregnancy Calculator Use Cases
The calculator is most useful for date orientation: checking how weeks and days relate to an EDD, understanding why IVF transfer dates use a different offset, or reconciling a known conception date with the conventional LMP clock. It can also show what gestational age a future calendar date would represent.
It cannot confirm pregnancy, assess fetal growth, diagnose symptoms, determine fetal sex, predict the actual birth date, or decide whether an EDD should be changed. Those questions require testing, clinical records, and qualified care.
- Estimate how many weeks pregnant a date represents.
- Calculate a due date from LMP and usual cycle length.
- Work backward from an established due date.
- Translate a known conception or ovulation date to gestational dating.
- Calculate a day-3 or day-5 embryo-transfer due date.
- See trimester and term-boundary dates on one calendar.
Accuracy, Clinical Dating, and Safety Limits
LMP dating assumes the reported period and modeled ovulation timing are reliable. Irregular cycles, recent hormonal contraception, postpartum cycles, uncertain bleeding, or variable ovulation can make that estimate less dependable. ACOG identifies first-trimester ultrasound measurement as the most accurate method to establish or confirm gestational age and recommends ART-derived dating for pregnancies conceived through assisted reproductive technology.
Once an EDD is established, later fetal size does not automatically become a new due date. Use the date recorded by the care team. Start prenatal care early, and seek medical care immediately for urgent maternal warning signs. CDC examples include trouble breathing, chest pain, fainting, severe persistent headache or belly pain, fever, heavy bleeding or fluid leakage, and thoughts of harming yourself or the baby.
- Do not use this page to decide whether a pregnancy is viable or whether symptoms are normal.
- Do not infer conception from symptoms or a positive test date alone.
- Use report-provided ultrasound gestational age rather than estimating it from an image.
- Use fertility-clinic records for IVF transfer date and embryo age.
- Contact a qualified healthcare professional about discrepancies or care decisions.
Helpful Pregnancy Dating and Safety References
These clinical and public-health references support the dating formulas, trimester boundaries, term labels, and safety language used on this page. They are provided for verification and further reading, not as substitutes for individual care.