One has to take into consideration that not every bitch follows the hormonal cycle according to the book! Research has shown that the ovulation peak can take place anywhere from day 7 until day 22 of the estrus cycle (3).
N B: It should be taken into consideration that not everyone takes into consideration as to when the estrus cycle of the bitch has taken place. Some bitches display little or no signs of estrus (vaginal discharge and interest in male dogs). It is therefore possible that a bitch in fact could be further with her estrus cycle than originally expected.