Last Menstrual Period (LMP)Calendar Date (First Day of Flow)
The calendar date marking the first day of normal bleeding of the last menstrual period. Under clinical obstetric convention established by Franz Naegele, gestational age begins on this date (approximately two weeks before biological fertilization actually occurs), establishing a standardized 280-day (40-week) timeline.
Default: 10 weeks prior to today
Menstrual Cycle LengthDays (Temporal Interval)
The total number of days from the onset of one period to the onset of the next. Clinical standard Naegele's rule assumes an unvarying 28-day cycle with ovulation on day 14. For cycles ranging from 21 to 45 days, ovulation timing shifts by (Cycle Length - 28) days, requiring mathematical compensation in the final EDD.
Default: 28 days (Normal range: 21–35 days)
Crown-Rump Length (CRL)Millimeters (Fetal Biometric Distance)
The direct ultrasonic measurement of the human embryo or fetus from the top of the cranial pole to the bottom of the buttocks. Between 6 and 14 weeks of gestation, CRL exhibits minimal biological variation and provides the gold standard for clinical gestational dating with a 95% confidence interval of +/- 3 to 5 days.
Default: Gestational age dependent (e.g. 17 mm at 8w1d)
IVF Embryo Transfer AgeDevelopmental Days (Cleavage vs Blastocyst)
The post-fertilization developmental stage of the embryo at the time of uterine transfer. Cleavage-stage embryos are transferred on Day 3 (requiring 263 days to EDD), whereas blastocysts are transferred on Day 5 (requiring 261 days to EDD) or Day 6 (requiring 260 days to EDD) to perfectly mirror natural implantation kinetics.
Default: Day 5 Blastocyst
Luteal Phase DurationDays (Post-Ovulatory Interval)
The biological duration between ovulation and the subsequent menstrual bleed, driven by corpus luteum progesterone secretion. While follicular phases vary widely, the normal luteal phase is remarkably stable at 12 to 14 days; deviations alter the calculated conception epoch relative to the LMP.
Default: 14 days (Normal range: 11–16 days)
Clinical Gestational Age (GA)Weeks and Days (Completed Chronology)
The official clinical age of pregnancy expressed in completed weeks plus days (e.g., 14w3d represents 14 completed weeks plus 3 days). Fetal developmental landmarks, viability thresholds (24w0d), and screening protocols are strictly governed by completed GA rather than calendar months.
Default: Calculated from dating anchor
