You can mark 'Start of new cycle' on the day of giving birth to start charting your first postpartum cycle (sometimes known as ‘Cycle 0’) which can be very long and runs through the return of ovulation (ROF) to the first postpartum menstrual period.
You can do this by scrolling back on the calendar screen and tapping the date when you gave birth then tapping 'Edit' in the dialog to go to that day on the data screen. Then tap to toggle on 'Start of new cycle' in the 'Fluid' or 'Stamps' section of your data screen.