The Fifth Child by Doris Lessing challenges our notions of gender roles and parenting. In a sense, Harriet, in her obsessive procreating, is making herself into something less than human, a woman who is merely a carrier of embryos instead of one living a fully human life. Next, her revulsion toward her fifth fetus violates the social norms of the society constructed in the book, and what are considered "natural" human instincts, and thus invokes the question of what is really human nature and what is merely societal norms. Although you could write about the notion of motherhood and what it means to be human, this would be a risky topic, as it touches too closely on many strongly held political opinions (e.g. the point at which a fetus is considered a living human being is still passionately debated).
The most obvious place to question what is human is in an analysis of Ben. A good approach might be to deconstruct our association of the human with the good. In other words, should the possibility that Ben tortures animals and commits rape make him less human? Or, in fact, are killing creatures one does not eat and committing rape uniquely human activities? Although the roots are the same, "human" and "humane" are not identical in meaning.
A third possibility would be to look at the notion of humanity in terms of race. Lessing herself grew up in Rhodesia (now Zimbabwe) under a system of apartheid in which whites considered blacks almost subhuman. Could Ben represent this racist view of Africans? (Was Ben really subhuman or mistreated?)
The Fifth Child relates to what it means to be human through its exploration of failure.
Until Ben is born, the life that the Lovatts lead is one of stereotypical success. Conservative values are echoed in a fruitful family, home in the suburbs, family togetherness for the holidays, and a life that others wish to emulate. Harriet and David are conscious of their desire for domestic bliss, a defining element of their success.
Where the element of failure becomes evident is in Ben's birth. From the time of her pregnancy, Harriet believes that what she is delivering is not really a child. She considers it "something" else. As Ben grows up, Luke, the eldest sibling, explains this to his siblings: “They are sending Ben away because he isn’t really one of us." Ben embodies the failure from which everyone runs: “Everyone in authority had not been seeing Ben ever since he was born. . . . Would people always refuse to see him, to recognize what he was?”
Harriet cannot run away from this failure. She must accept this as part of her identity. She had this entire vision of a large family, domestic happiness, and an ideal of perfection embedded in her mind. She never considered the possibility of failure. Her "ruthlessness," as her sister puts it, compels her to institutionalize her son. When this does not work, she spends an increasing amount of time trying to nurture him into a success. This comes at the detriment of the attention she can dedicate to her other children. In this light, Harriet has to wrestle with multiple levels of failure. Harriet never envisioned that her dreams of success could resemble such a nightmare.
Lessing's work suggests that human identity means having to accept that a degree of failure lies within all of our hopes and dreams. We strive for the garden and sometimes wind up in the desert. The vision that Harriet and David hold, one that is transmitted to their children, only anticipates success. Ben's arrival forces failure to be acknowledged. Harriet's wondering about Ben at the end is the ultimate representation of her own shortcomings. She does not know about the future of her "alien" son. All she knows is that her son is an embodiment of her failure and his own.