Ovulation is the release of an egg. Conception is fertilization of that egg. They are often a day apart, and sometimes people use the words for the same event. For a due-date estimate the difference matters by about one day, which is why both dates use a 266-day count on this site.
None of these dates confirm that conception happened. A calculator cannot rule a pregnancy in or out.
The order of events
In a textbook 28-day cycle, the next period is predicted 28 days after the last one, and ovulation is estimated 14 days before that next period. The ovulation calculator uses that rule and a fertile window of several days before ovulation, because sperm can survive longer than the egg.
The egg is typically available for about 12–24 hours after release. Fertilization, when it happens, falls in that short span. Implantation into the uterus is later, often about 6–10 days after ovulation. Implantation is not the date you add 266 days to.
What to type into a due date tool
| Date you know | Add | Tool |
|---|---|---|
| First day of the last period | 280 days, adjusted if the cycle is not 28 days | Pregnancy due date calculator |
| Ovulation | 266 days | Due date by ovulation |
| Conception | 266 days | Same ovulation tool |
A worked pair is in due date from ovulation or conception: 14 March versus 15 March moves the estimate from 5 December to 6 December.
Why the period method is two weeks ahead
Gestational age starts at the period, not at fertilization. In the 28-day model you are already about two weeks “pregnant” at conception. How many days a pregnancy is is that 280 versus 266 gap. How a due date is calculated is the period formula. If the dates you have are approximate, how accurate due dates are is the reason to treat the result as a window.