Guides and explanations
The arithmetic behind each calculator, written out in plain language, including the parts where the estimates stop being dependable.
How is a pregnancy due date calculated?
Where the 280-day rule comes from, why pregnancy is dated from your last period, and how cycle length and ultrasound change the estimate.
5 min readHow many weeks pregnant am I?
How pregnancy weeks are counted, what 12w3d means, how trimesters are divided, and why weeks do not convert neatly into months.
4 min readHow does an ovulation calculator work?
Why ovulation is estimated by counting back from your next period, how the luteal phase makes that work, and where the method breaks down.
5 min readWhat is the fertile window?
Why the fertile window is about six days long, which days carry the highest chance of conception, and why no day counts as a safe day.
4 min readWhen should you take a pregnancy test?
Why hCG only appears after implantation, what makes early tests unreliable, and which date gives a negative result real meaning.
5 min readCan your due date change?
Why due dates are often revised after an early scan, when a change is routine, and why late-pregnancy scans are not used for dating.
4 min readHow is an IVF due date calculated?
The transfer-day formula for day 3, day 5 and day 6 embryos, why frozen transfers use the same rule, and why gestational age starts at two weeks.
4 min read