Menstrual cycle & ovulation
Clinics use cycle day 1 (CD1) to time blood tests, scans and the start of medication. Spotting beforehand usually does not count — your clinic will tell you how it defines day 1 and whether to call them.
Also known as: CD1, day 1, day one of your period
Planning tools
Sources
Last reviewed: 23 September 2026. Written by the Antega Editorial Team. General information only — always follow the advice of your own clinic, doctor or pharmacist.