Ron had always assumed that women became crazy at around twelve. About the time they started getting boobs, and about the time he had started noticing their existence. Convenient.
Ginny was the exception to this rule. She had never been crazy. She had been scary, but there was always a reason for her anger. That changed when she got pregnant. Naturally, Ron assumed that this meant she would be normal again after she had the kid. Bad assumption.
Hermione was the very reason he had established this theory in the first place. From the moment he realized what her being a girl meant, he knew she was crazy. To tell the truth, she was so funny when she was mad, that he developed a rather bad habit of pissing her off just to hear her huff "Ronald!" and stomp off.
Rose disproved this theory once and for all.
When she was a baby, she cried for no reason at all. Hermione, as exhausted as he was, kissed his cheek and assured him that babies did that. Ron loved his baby girl with all his heart, but he did wish that she might stop crying when there was absolutely no reason to be upset. But he would dutifully offer a bottle, check her diaper, sing a lullaby, and rock her gently until the fussy baby went back to sleep.
When Hugo came along, Rose was two, but still had a tendency to be upset about nothing. Ron prepared himself for dealing with two upset children and no means of calming either. But he was surprised. Every time Hugo cried, he went through the routine. And every time, he found the source. Hugo never seemed to cry "just because." Or he did very rarely.
Upon this discovery, Ron decided that all females were completely insane, regardless of age or maturity level. There was no rhyme or reason to their crazy, no way to predict it, and no way to protect oneself from it. He just had to pray he dodged it, and ride it out when he failed.
But, Merlin, he loved his crazy women.