Fans of the Dragon Ball franchise know that Goku missed Goten's birth, but it turns out, he also missed the birth of Gohan. However, it was also revealed in Dragon Ball Super that this could very well be a peculiar family trait and not just negligence. Goku's a great warrior, but his knowledge of the world only goes so far.
The Cell Games Saga caused one of Goku's many deaths. He sacrificed himself during the tournament and he returned after seven years. This meant that he missed his son, Goten's birth—particularly since he wasn't even aware that Chi-Chi was pregnant. He finally met Goten after those many years. However, an easy-to-miss conversation in Dragon Ball Super reveals that he also missed Gohan's birth, though there doesn't seem to be an official reason thus far.
Dragon Ball Super Chapter 30: The Man Named Jiren features the birth of Vegeta and Bulma's daughter, Bulla. Vegeta declined to participate in the Zeno tournament because he wanted to be present for the birth. Whis then uses his magic abilities to deliver Bulla early, transporting her out of Bulma's womb. This negates Vegeta's given excuse, but a comment from Goku reveals that he actually doesn't know how children are born.
Goku says, "Wow! I've never seen a baby born before! So that's how Chichi did it." While this comment may seem rather normal for Goku—who is often clueless about some aspects of the world—it has a deeper meaning to it. As far as fans know, Goku was not dead for Gohan's birth. The manga featured a timeskip, showing Gohan when he was already a little older. He was supposed to be born a year after Goku and Chi-Chi's marriage, which occurred after the end of the 23rd World Martial Arts Tournament. Since these moments were not fully shown and Goku was presumably alive, fans likely assumed that Goku would have been fully present at Gohan's birth. He has no excuse not to be. However, as it stands, he was not (or if he was), he was outside of the room where it occurred and was brought in after the birthing was over—never realizing how children are actually born.
It is entirely possible that Goku might not have been present if Dragon Ball adheres to some aspects of Japanese culture. Some hospitals do not allow fathers in the room with expectant mothers, while others do. Sometimes classes are offered to fathers that teach them about their role during the birth process and require them to stay by the mother's head—no lower. It is not a guarantee that a father will be present in the room, though he may be nearby in a waiting room. Based on this, it definitely seems likely that this could have been the case for Goku. However, Dragon Ball Super Chapter 77: Bardock, Father of Goku, reveals that Goku's father missed his birth also, because he was away, training. It is then also possible that Goku could have been just like his father and been briefly away when Gohan was born, carrying on a family tradition he isn't even aware of. Regardless of why, the fact is Goku was not present for either Gohan or Goten's births and as a result, this Super Saiyan has no idea how children are born despite having two of his own.