Frequently asked questions
How accurate is a conception date estimate?
A conception date estimate is accurate to within about 5 to 7 days. Sperm can survive up to 5 days in the reproductive tract, so intercourse several days before ovulation can result in pregnancy. The exact moment of fertilization is rarely known unless IVF was used.
Why does this calculator subtract 266 days instead of 280?
The 280-day figure counts from the last menstrual period (LMP), which is about 14 days before conception. Actual gestation from conception to birth is approximately 266 days (38 weeks). This calculator works backward from the result (due date or birth), so it uses the 266-day figure.
Can I determine the exact date of conception?
No, not without IVF records. Natural conception involves variables: sperm can live 3 to 5 days, and the egg survives 12 to 24 hours. This means conception could have resulted from intercourse up to 5 days before ovulation. The calculator gives the most probable ovulation/fertilization date.
Does this work if the baby was born early or late?
The 'from birth date' mode assumes a full-term delivery (38 weeks from conception). If the baby was significantly premature (e.g. born at 34 weeks), the actual conception date would be later than calculated. Adjust by adding the weeks of prematurity. For overdue babies, subtract the extra days.
Is the fertile window the same as the conception date?
No. The fertile window is the range of days when intercourse could have led to pregnancy (about 5 days before to 1 day after ovulation). The conception date is the single day fertilization most likely occurred (ovulation day). Intercourse anywhere in the window could be responsible.
Can paternity be determined from conception date?
A conception date estimate narrows the window to about a week, which may help rule out some possibilities. However, it is not precise enough to determine paternity conclusively. DNA testing is the only reliable method for confirming paternity.