Over the course of A Thousand Splendid Suns, Mariam and Laila gradually become more intimate, forming a close relationship akin to that of a mother and daughter. Mariam's motherly behavior towards Laila emerges as a motif in the novel, characterizing much of her interaction with the other woman. Note the following passage from Chapter 39, in which Mariam advocates for a belabored Laila at the hospital: 

 Mariam waded in. She dug in her heels and burrowed against the elbows, hips, and shoulder blades of strangers. [...] To propel herself forward, Mariam clawed at necks, at arms and elbows, at hair, and, when a woman nearby hissed, Mariam hissed back. Mariam saw now the sacrifices a mother made. Decency was but one. [...]
"My daughter's water broke and the baby won't come," Mariam called.

Mariam refers to Laila as her daughter; she also claims to now understand "the sacrifices a mother made." While this claim could be a reflection on Laila's sacrifices, it can also be interpreted as a reflection on the sacrifices Mariam makes for Laila. The older woman claws away at the crowd of patients, forcing herself to the front of the line to get her surrogate daughter medical care.  

Mariam sacrifices much more in due course, eventually killing Rasheed to save Laila's life and enable her to escape with Tariq. In the following excerpt from Chapter 46, Mariam reassures Laila about her decision to stay behind in Kabul and shoulder the blame for Rasheed's murder:

"When [the Taliban does capture us], they'll find you as guilty as me. Tariq too. I won't have the two of you living on the run, like fugitives. What will happen to your children if you're caught?" Laila's eyes brimming, stinging. "Who will take care of them then? The Taliban? Think like a mother, Laila jo. Think like a mother. I am."

Mariam implores Laila to "think like a mother," considering what is best for Aziza and Zalmai. Mariam herself thinks like a mother—Laila's mother—in the midst of their predicament. She is calm, rational, and focused on what is best for Laila and her children, even if that picture of the future does not contain Mariam.