We learn about the duties of Birthmothers in chapter five, when Lily expresses the wish to become one. Her mother is quite upset by her declaration and responds tersely, telling her that she should not say that for there was "little honor in that Assignment."
Lily then tells her mother what she had been informed by Natasha, a Ten who lives around the corner and spends volunteer hours at the Birthing Center. In terms of this, it appears that Birthmothers are given good food, have very gentle exercise periods and generally "just play games and amuse themselves while they're waiting" to give birth.
In these terms, then, it seems as if Birthmothers have very pleasant jobs. Once they are pregnant, they are taken good care of and just laze about. They are not expected to perform any labor. It is these aspects which Lily finds attractive.
Her mother informs her, however, that Birthmothers only enjoy these privileges thrice in their lifetimes. After giving birth a third time, they become Laborers and have to perform hard physical tasks until they are aged. Once they are too old to work, they enter the House of the Old.
The prospect of having to work hard for the rest of her life after three years of advantage makes Lily reluctantly agree that such a life does not sound so appealing after all. Lily's father also tells her that Birthmothers do not see their babies after birth, which means they are not allowed to nurture their offspring or bond with them.
It is obvious from the above that being a Birthmother requires emotional tenacity, for they should be able to deal with being separated from their newborns immediately after having borne them for nine months. It must be difficult. They are mere tools in this harsh and unforgiving society. Furthermore, once their usefulness is over, they are demoted to the lowest Assignment. In effect, their only real reward is the knowledge that they have contributed to the growth of their community.
It is for this reason that Lily's mother is so upset about her desire to become a Birthmother for there is clearly no honor in being one.
In conclusion, the passage makes it ironically clear that this supposedly utopian society has a clinical, harsh approach to life. It is devoid of emotion, which we learn is discouraged. Instead of the newborns being nurtured by their birth mothers, they are taken care of by nurturers who would obviously not have the close bond that the mothers would have had with them. In addition, it emphasizes the fact that members in this society are raised to be generally devoid of emotion -- therefore, there cannot be any real love or care. Relationships are based on mere predetermined attachments.