Serena's statement reflects how she knew she was "different" from other women. She knew that she was not meant to be a mother.
Serena's words show an acceptance of her fate. She had just recovered from passing out and losing a great deal of blood. Pemberton tells her that she lost the baby she was carrying and would not be able to have another one. Rash's narration shows that Serena was not saddened or emotionally devastated by the news. Rather, she understood that it was meant to be:
Serena remained silent for almost a minute, and Pemberton wondered if the drugs were taking hold again. Then Serena took a breath, her mouth kept open as though about to speak as well, but she did not speak, not at that moment. Instead, Serena closed her eyes and slowly exhaled, and as she did her body seemed to settle deeper into the mattress. Her eyes opened.
Serena says that her body "knew all along." She means that she knew that being a mother and delivering a baby was not meant for her. She recognized that the traditional role of motherhood was not going to be for her. When she tells Pemberton that "Your blood merged with mine," and "That's all we ever hoped for anyway," it shows how Serena did not define herself through motherhood. She saw her life as being different from the conventional standard to which most women were held. Serena knows that the traditional path was not meant for her. Serena was always meant to be different, something her body "knew all along."