Or HaChaim on Exodus 1:15/אור החיים על שמות א׳:ט״ו

1 ויאמר מלך מצרים למילדות, The king of Egypt said to the midwives, etc. So far the Torah had described everything that happened in the plural, i.e. "they imposed taskmasters," "they enslaved them," etc. The king enjoyed a consensus of his people for his action (perhaps because the discrimination against the Jews had not yet been enshrined in law, but was "merely" a clever ruse to disenfranchise part of the population). Now, however, the king alone resorted to a measure which would not become public knowledge and would not run the risk of the Jewish mothers trying to hide the fact that they were pregnant so as to mislead the midwives. I will go into details later.

2למילדות העבריות, to the Hebrew midwives, etc..It is interesting to surmise what exactly the king said to these midwives the first time. [remember that the Torah credits the king with a second אמירה in the next verse. Ed.] Perhaps the Torah has clued us in with the words: "the name of one was Shifrah, whereas the name of the other one was Puah." No doubt there had been numerous midwives who attended the Jewish women. The king did not bother to know any of them by name except those whom he entrusted with a specific task. The Torah tells us that the king spoke to them by name, thus selecting them to perform a Royal command. By calling them by name, the king elevated them to the status of "midwives -in-chief." He had to commence his instructions by saying: "when you deliver the Hebrew women, etc." meaning that the order he was giving applied not only to those two but to all the Hebrew midwives. He instructed them directly as a sign of promoting them over their colleagues.

3We encounter the expression אמירה as one which describes appointing someone to a high position in Deut. 26, 17-18 where Israel and G'd each elevate the other to an exclusively superior status. Israel had appointed G'd as its only G'd, and in return G'd appointed Israel as His chosen people. Perhaps the fact that the midwives risked the displeasure of Pharaoh as well as being demoted or worse is reflected in the reward G'd gave them as reported by the Torah in verse 21.