Conception CalculatorLast Menstrual Period Pregnancy Due Date Formula
LMP due date = first day of the last menstrual period + 280 days + (average cycle length - 28 days). The adjustment moves the estimate later for a longer average cycle and earlier for a shorter one. Estimated ovulation and conception = LMP + cycle length - 14 days.
The formula assumes the entered LMP and cycle average describe the pregnancy cycle. It cannot identify atypical bleeding, an anovulatory cycle, or ovulation that occurred at an unexpected time. The interface accepts average cycle lengths from 21 through 45 days and labels every output as an estimate.
The 280-day convention and the role of early ultrasound are described in ACOG Methods for Estimating the Due Date.
Conception Date and Reverse Due Date Calculation Formula
Known conception due date = conception date + 266 days. Reverse conception estimate = known due date - 266 days. Equivalent LMP = due date - 280 days, or conception date - 14 days. These paired formulas keep gestational age on the standard 40-week scale.
A conception result includes a central date plus two days on either side. The broader possible intercourse window starts five days before and ends one day after the central estimate. These ranges communicate biological uncertainty; they are not paternity evidence and do not identify the exact timing of fertilization or implantation.
Ultrasound Gestational Age and Pregnancy Dating Formula
Equivalent LMP = ultrasound date - reported gestational age in days. Estimated due date = ultrasound date + (280 days - reported gestational age). For example, a scan reported as 9 weeks 3 days represents 66 elapsed gestational days, which are subtracted from the scan date.
The calculator accepts only the gestational age supplied by the user; it does not derive age from crown-rump length, images, fetal measurements, or a report narrative. Ultrasound gestational ages are limited to 0 weeks 0 days through 42 weeks 6 days. A clinician must decide whether a scan should establish or revise an official due date.
IVF Embryo Transfer Due Date Formula for Day 3, 5 and 6
IVF due date = embryo transfer date + 266 days - embryo age. This produces transfer + 263 days for a day-3 embryo, +261 days for day 5, and +260 days for day 6. Estimated fertilization = transfer date - embryo age. Equivalent LMP = estimated fertilization - 14 days.
Fresh and frozen embryo transfers use the same arithmetic because the formula depends on the recorded transfer date and embryo age. The calculator does not infer embryo age from a treatment description. ACOG recommends that pregnancies resulting from assisted reproductive technology use ART-derived gestational age to assign the estimated due date.
Ovulation Date and Six-Day Fertile Window Formula
Estimated ovulation = cycle start + cycle length - 14 days. The fertile window runs from five days before estimated ovulation through ovulation day. Peak days are displayed as the two days before ovulation plus the estimated ovulation date. Each later forecast cycle advances by the entered average cycle length.
This six-day definition aligns with the ASRM committee opinion on optimizing natural fertility and the Office on Women's Health ovulation calculator. Neither source implies that a calendar can confirm ovulation in an individual cycle.
Pregnancy Weeks, Trimesters and Delivery Window Calculations
Gestational age in days = today - equivalent LMP. Complete weeks are the whole-number result of dividing elapsed days by seven; the remainder is displayed as additional days. The timeline calculates reference dates at weeks 12, 20, 28, 37, and 40 from the equivalent LMP.
The estimated delivery window shown by the tools runs from 37 through 42 gestational weeks. It is included to prevent the 40-week date from appearing to be a guaranteed delivery day. It is not clinical advice about term definitions, monitoring, induction, or when delivery should occur.
Pregnancy Quiz and Home Test Timing Rules
The pregnancy quiz does not estimate the probability of pregnancy from symptoms. For a predictable cycle, the first testing checkpoint is the expected period date: LMP + average cycle length. For irregular or uncertain cycles, the conservative checkpoint is the later of LMP + 36 days or the most recent possible conception date + 28 days when those dates are known.
A negative result before that checkpoint is labelled as potentially early. A later negative result leads to a repeat-test reminder in about one week. Positive and invalid results bypass calendar scoring and lead directly to confirmation or retesting guidance. These rules follow FDA, Office on Women's Health, and MedlinePlus patient information and do not diagnose pregnancy.
UTC Date Handling, Validation Rules and Supported Date Range
Inputs are parsed as calendar dates and normalized to UTC date-only values before arithmetic. This prevents daylight-saving transitions and local time-zone offsets from moving a displayed result backward or forward by one day. The calculations use whole calendar-day offsets rather than elapsed local hours.
Supported input years are 1900 through 2200. Future LMP, known conception, and ultrasound dates are rejected when they would describe an event that has not happened. Future due dates and planned IVF transfer dates are allowed where they support legitimate planning. Impossible dates and incomplete inputs return a field-level error instead of a result.
How to Verify a Pregnancy Calculator Result
First, confirm that the input is the first day of bleeding rather than the last day of a period, that cycle length counts from one period start to the next, and that ultrasound age belongs to the scan date entered. For IVF, check the transfer date and embryo age against the clinic record.
Next, compare the output with the date documented by the care team. A matching result shows that the same calendar formula was likely used; a mismatch does not prove that either record is wrong. Report reproducible arithmetic issues with non-sensitive sample dates on the calculator corrections page.