Correct me if I'm wrong but... a viable pregnancy can be achieved whilst someone is having a period IF their cycles are short/irregular, such as may be seen with breastfeeding. Consider the following:
- Woman's cycle is 20 days long, she bleeds for 6 days (days 1 -6).
- Ovulation occurs 12-16 days before onset of period, in a shorted period of 20 days this would occur at day 5 - 9 of cycle, or whilst she is still bleeding.
Sure it wouldn't happen often, but the human body is anything but textbook. The complex interplay of hormones should never be presumed to be straightforward, especially whilst breastfeeding.
- Woman's cycle is 26 days long, she bleeds for 5 days (days 1 - 5).
- Ovulation occurs 12 - 16 days before onset of period, which would be days 10 - 14.
- Woman has had sex on day 5, sperm are viable 5 days later on the first day of ovulation.