1. She probably has to pee, right this moment.
2. Her mood may change at any moment.
... and then change back. Then back once more.
3. Morning sickness does not just occur in the morning.
Some women experience morning sickness all day, or feel worse into the evening. Be sympathetic.
4. She looks at you differently now.
This can work both ways, if you actively engage in preparing for the baby, she may love you more deeply. If you don't, she may be weary of your suitability as a father.
5. It takes a fraction of the time for her to get upset that it previously did.
Probably about a quarter of the time it used to.
6. You will hear the female anatomy discussed with unparalleled candor.
It will make you uncomfortable. Very uncomfortable. You won't look at women the same way.
7. She will get freaked out.
Probably by some article she read, it is your job to find another article that says the first article is wrong.
8. If you don't sympathize with her, you will drop in her estimation.
Demonstrate some empathy for her aches, pains, nausea and fears or your relationship will be strained.
9. Don't expect her to maintain her pre-pregnancy activity levels.
She tires easily and won't be able to keep up with her normal activities, from her job to anything she might do around the house. You will have more to do.
10. She has to pee again.