As the first trimester comes to a close, the hormone progesterone triggers a change in your blood vessels: They dilate, or expand, allowing the increased blood flow your fetus needs to get all this work done. Those dilated vessels cause a drop in blood pressure, which in turn can make you feel dizzy or faint.
This shouldn't cause a big problem, but it's something for your partner to be aware of. A shoulder to lean on or a supportive hand (or cold washcloth) might come in handy at this time.
Another bit of knowledge that will do everybody some good? Pregnancy affects sex drive.
Obvious, perhaps, but some people don't realize that not everyone responds to pregnancy hormones in the same way. Partners, be aware: Your woman might go either way on this one, and, around this time, aversion is probably a more common response. Her boobs hurt, she's dizzy, heartburn is searing her chest and her waist is going bye-bye. Don't feel too hurt if she's not up for it. You'll have her back soon enough (very soon, perhaps -- many women report increased libido in the second trimester).
Make the effort to connect physically in other ways -- perhaps a massage, or a good, long hug, or a snuggle-on-the-couch movie night ("Knocked Up," anyone?). It's not sex, but it's nice.
Some other things to think about during week 12 …