Day 1?

I'm waiting for my period to start so I can start birth control pills on day 3 in preparation for IVF. Just when it's important to get the Day 1 correct, my body plays a trick on me. Not only did my period started later yesterday afternoon but it was very little. Then it came full force late last night. I didn't know whether to consider July 26th as day 1 or July 27th as day 1.

I called the patient co-ordinator and she confirmed that July 27th is day 1 since the full flow was much later in the day.