"Ubnei Yisrael paru vayishritzu vayirbu vyatsmu." The passuk uses many terms to describe Bnei Yisrael's growth. The Bostoner Rebbe zt"l, Reb Levi Yitzchak Howoritz, in his sefer Shaar Levi, asks: Why does the passuk need to describe this so deeply, to go so far as to use the word "vayishritzu," which is a less refined terminology than it needed to use? He explains that the term "peru" refers to the etzem maaseh mitzva, or physicial aspect of the mitzva, which determines the "quantity" of the children, whereas the term "revu" refers to the "quality," which depends on the thoughts and respect a person has for the mitzva. Through this idea of "revu," a person can turn even the gashmius into ruchnius. The Zohar and the Ramban similarly discuss that a child's character is dependent on how properly his parents performed their mitzva of peru urevu, meaning not only the phyiscal--peru, but also the thoughts and mindset--revu.
This is what the passuk means by "Ubnei Yisrael paru vayishritzu vayirbu vyatsmu," Paru--they fulfilled the physical aspect of the mitzva, and vayishritzu--they begot children lacking in proper character traits. Vayirbu--they also fulfilled the machshava aspect of the mitzva, and vyatzmu--they became strong, meaning righteous and proper children. Both groups of children were born to Bnei Yisrael at this time.
This is an important reminder that the shemiras habris that we struggle with on a daily basis isn't something given to us purely as a nesayon. It is in fact the opposite. It is something we are meant to make holy and pure, and elevate for Hashem. As Chizkuni and the Netziv in Ha'amek Davar discuss, Hashem brought us to Mitzrayim specifically to make us into a great nation and fulfill the bris with Avraham. He is on our side, on a constant basis. What's better than that?