Pregnancy Conception Date & Fertile Window Calculator
Estimate probable fertilization dates, trace the 6-day intercourse fertility window, and track embryonic milestones from LMP, ultrasound, or due date.
Calculation Inputs
Dates calculated are estimates only, assuming standard follicular development and ovulation lengths. Early ultrasound scans performed in the first trimester remain the clinical gold standard for establishing true gestational age. Always consult your obstetrician for clinical decisions.
Estimated Conception Date
October 2, 2026
Probable Conception Fertile Window
September 27, 2026 – October 2, 2026
Intercourse during these dates likely led to fertilization, factoring in the life cycle of sperm and egg.
Due Date (EDD)
June 25, 2027
Gestational Age
6 Weeks, 0 Days
Days Remaining
238 Days
Developmental Milestones Calendar
| Milestone Event | Gestational Age | Target Date | Status |
|---|---|---|---|
Last Menstrual Period (LMP) Gestational dating clock begins based on reference period cycle. | Start | September 18, 2026 | Completed |
Estimated Conception Fertilization of the egg by sperm in the fallopian tube. | 2 Weeks | October 2, 2026 | Completed |
Blastocyst Implantation Embryo attaches tightly to the uterine lining. | 3.2 Weeks | October 11, 2026 | Completed |
Neural Tube Closes Early brain and spinal cord form and fuse shut. | 4 Weeks | October 16, 2026 | Completed |
Fetal Heartbeat Heartbeat is detectable on early ultrasound. | 6 Weeks | October 30, 2026 | Completed |
Embryo to Fetus Transition Basic organogenesis complete. Organ systems start working. | 10 Weeks | November 27, 2026 | Upcoming |
Anatomical Screening Window Mid-pregnancy ultrasound to check structural developments. | 19 Weeks | January 29, 2027 | Upcoming |
Fetal Viability Medical threshold of potential survival outside the womb. | 24 Weeks | March 5, 2027 | Upcoming |
Full Term Status Baby's lungs and brain are fully mature for safe birth. | 37 Weeks | June 4, 2027 | Upcoming |
Estimated Due Date (EDD) Estimated completion of 40 weeks gestation. | Due Date | June 25, 2027 | Upcoming |
Obstetric Chronology, Naegele's Rule & Embryonic Aging
Understanding gestational vs conceptional dating and fetal developmental milestones
In clinical obstetrics, pregnancy dating establishes critical management windows for prenatal aneuploidy screening, maternal serum analyte interpretation, and timely delivery scheduling. A fundamental distinction exists between gestational age (dated from the first day of the Last Menstrual Period, averaging 280 days or 40 weeks) and conceptional or embryonic age (dated from actual oocyte fertilization, averaging 266 days or 38 weeks).
The historical standard for estimating delivery dates from menstrual history is Naegele's Rule. Introduced by German obstetrician Franz Karl Naegele in the 19th century, it calculates the Estimated Due Date (EDD) by adding 1 year, subtracting 3 months, and adding 7 days to the first day of the LMP:
EDD = LMP + 1 Year - 3 Months + 7 Days (or LMP + 280 Days)
Conception Date = EDD - 266 Days (or LMP + Cycle Length - 14 Days)Because fertilization must occur during the viable lifespan of the ovulated secondary oocyte, calculating conception simultaneously establishes the key gestational milestones:
| Developmental Period | Gestational Age (from LMP) | Embryonic Age (from Conception) | Key Morphological Milestones |
|---|---|---|---|
| Fertilization & Cleavage | 2 Weeks 0 Days | Day 0 (Conception) | Zygote undergoes mitotic cleavage forming morula and blastocyst |
| Uterine Implantation | 3 Weeks 0–3 Days | 6 to 10 Days Post-Conception | Syncytiotrophoblast invades endometrium; human chorionic gonadotropin (hCG) rises |
| Embryonic Heartbeat | 5 Weeks 5 Days to 6 Weeks | 3 Weeks 5 Days | Primitive tubular heart initiates rhythmic contraction; detectable on transvaginal scan |
| First Trimester Completion | 13 Weeks 6 Days | 11 Weeks 6 Days | Organogenesis complete; transition from embryo to fetus; placenta assumes full endocrine support |
| Fetal Viability Threshold | 24 Weeks 0 Days | 22 Weeks 0 Days | Alveolar surfactant secretion begins; neurological reflexes; intensive neonatal survival potential |
| Full Term Birth | 39 Weeks to 40 Weeks 6 Days | 37 to 38 Weeks | Complete pulmonary lung maturity and thermal subcutaneous fat accumulation |
Clinical & Obstetric Frequently Asked Questions
Rigorous medical answers regarding conception timing, gestational age, and ultrasound dating
How does clinical gestational age differ from true embryonic conception age?
Gestational age is dated from the first day of the last menstrual period (LMP) and totals 280 days (40 weeks) at full term, incorporating the roughly two weeks prior to ovulation. Embryonic conception age measures fetal development from the exact day of fertilization, totaling 266 days (38 weeks).
How does Naegele's rule calculate estimated due dates from menstrual records?
Naegele's rule calculates the Estimated Due Date (EDD) by taking the first day of the last menstrual period, adding 1 year, subtracting 3 months, and adding 7 days. This assumes a regular 28-day cycle with spontaneous ovulation occurring on cycle day 14.
Can an early first-trimester ultrasound override menstrual dating?
Yes. The American College of Obstetricians and Gynecologists (ACOG) guidelines state that when early crown-rump length (CRL) ultrasound measurements prior to 14 weeks diverge from LMP dating by more than 5 to 7 days, the ultrasound gestational age becomes the authoritative clinical benchmark.
What defines the 6-day intercourse window leading to conception?
The conception window spans the five days prior to follicular ovulation plus ovulation day itself. Healthy spermatozoa maintain fertilizing capacity in cervical crypts for up to 120 hours, whereas the ovulated secondary oocyte survives for only 12 to 24 hours post-release.