Day one is the first day of a period
Most cycle calculations start with the first day of menstrual bleeding. An average cycle length is then used to estimate the next period. A single month may not represent your usual pattern, so an average from several cycles can be more useful.
Ovulation is estimated backwards
Ovulation is often estimated by counting backwards from the expected next period. It is often estimated by counting backwards from the expected next period, rather than assuming it always occurs on day 14.
Why the fertile window is wider
The most fertile days surround ovulation because sperm can remain viable for several days and the egg has a shorter lifespan. A calendar can only estimate this window; it cannot confirm that ovulation occurred.
How pregnancy due dates are estimated
A standard estimated due date is about 40 weeks from the first day of the last period or about 38 weeks from conception. Pregnancy length varies around the estimated date, and clinical dating may revise the calculation. Dating scans and clinical information can revise the estimate.
Important limitations
- Irregular cycles make calendar estimates less reliable.
- Hormonal contraception, breastfeeding, illness and perimenopause can alter timing.
- These tools do not confirm pregnancy, ovulation or fertility.
- Calendar estimates should not be the only method used to prevent pregnancy.