Genesis 16:2

And Sara said to Abram, Behold, now, the Lord hath restrained me from bearing, go to my handmaid and set her free; perhaps I may be builded by her. And Abram hearkened to the word of Sara.

AND ABRAM HEARKENED TO THE VOICE OF SARAI. Scripture does not state, “And he did so.” Rather it says that he hearkened to the voice of Sarai, thus indicating that even though Abram was very desirous of having children, he did not do so without permission of Sarai. Even now it was not his intention to build up a family from Hagar, and that his children be from her. His intent was merely to do Sarai’s will so that she may build a family from Hagar for she will find satisfaction in her handmaid’s children, or that by the merit of this act she will become worthy to have children, as our Rabbis have said.
Scripture further says, And Sarai took, to inform us that Abram did not hurry the matter until Sarai took Hagar and gave her to him. Again, Scripture mentions Sarai Abram’s wife… to Abram her husband, to allude that Sarah did not despair of Abraham and that she did not render herself distant from him as she was his wife and he her husband, but she wanted that Hagar also be his wife. This is why the verse states, And she gave her to Abram her husband to be his wife, meaning that she was not to be as a concubine but as a woman married to him. All this reflects the ethical conduct of Sarah and her respect towards her husband.

הנה עצרני ה' מלדת, "even though G'd has promised to grant you offspring to whom I will give this land, (Genesis 12,7) He has not said that the mother of Avram's offspring would be I."

אולי אבנה ממנה, Sarah hoped that the jealousy which would develop within her when she saw that Hagar gave birth to a child for Avram would stimulate her own biological and sexual urges so that in due course she too would become pregnant by her husband. (based on Megillah 13)

שרי לקול, the Torah wants to make plain that Avram was not motivated by sexual lust to sleep with Hagar, but had only fallen in with Sarai's reasoning, hoping to assuage her feelings in the matter.

ותאמר ... עצרני ה' מלדת, she meant that seeing that G'd had prevented her from giving birth during all these years, she had now abandoned hope of ever bearing a child as she had aged in the interval.

אבנה, the son from this union would be called אבנה, "I shall be built up." All children are a building consisting of genetic input by father and mother. Sarai said that any son from this union with her husband would be accepted by her as if he were part of her biological family. She would treat him as her own son.

וישמע אברם, he agreed with Sarai's plan, preferring not to wait any longer for G'd to fulfill His promise in giving him children from Sarai as he had understood this promise up until now. He decided that since Sarai had stopped menstruating she would not be able to become pregnant. As a result, he reasoned that G'd's promise to him was to be fulfilled via Hagar who would become the mother of any children of his. Seeing that even Sarai had arrived as such an interpretation of G'd's promise, he had no reason to disagree with her. This is why Avram did not pray to G'd asking Him to make Sarai bear him a son, as opposed to his son Yitzchok later on who did pray to G'd for Rivkah to bear him a son. It is also possible that Avram did pray concerning this matter, but that he had concluded that G'd had seen fit to deny his request. In that event, we must understand G'd's delay in answering Avram's prayer as being designed to show that He could make even a ninety year old barren woman become pregnant and give birth to a healthy child.