How the due date is calculated

• Last period: first day of your last period + 280 days (40 weeks), adjusted for your cycle length (Naegele's rule).
• Conception date: + 266 days (38 weeks).
• IVF: transfer date + 263 days for a day-3 embryo, or + 261 days for a day-5 blastocyst.

Your doctor may adjust the date after your first ultrasound scan.