I just wanted to add a man's voice to this thread, because you search for pregnancy and impotence, this page is one of the top results. I describe my specific problem below, but I want to say that if this problem persists for many months or the ED problems exist outside of just the fertile time, go see a doctor. ED can be a symptom of other serious health issues.
I suffered from psychological impotence when my wife and I were trying. I am 30 and relatively healthy, and I've never, ever had any type of ED problem -- in fact the opposite, if that makes sense. Anyway, when my wife and I started trying, something changed, and I started to experience the same thing mentioned above: I'd start out OK but then very quickly lose my erection.
I'm not sure why. I love my wife and find her sexually attractive, and I look forward eagerly to being a father. (I don't think I have any subconscious issues in that area, at least none more than the average guy.) But for some reason, I started having this problem, and then once you *start* having that problem, it's always in the back of your mind and starts to build anxiety and feelings of inadequacy. Thoughts like "What's wrong with me? This is the one thing every man should be able to do and I can't do it? Etc."
My advice to the woman:
1) BE SUPPORTIVE. Do not criticize him. If he's anything like me, he's in a very vulnerable state.
2) Don't worry about it too much. It probably has nothing to do with you or his feelings about you or his desire to have children.
3) If you're young-ish and have plenty of time to keep trying, don't make a big issue of your man's problems. Don't rush him to the doctor. Just relax and go with it, and it will probably work itself out. If time is running out, I'm not sure the best course of action; consult your doctors.
4) If you're not already, start having *lots* of sex outside of the fertile window. Get into a good habit of lovemaking just for the fun of it, regardless of baby making. This will repair your guy's sexual confidence, which is surely shaky if he's having problems, and it will be easier to keep him ignorant of the fertile time, if you decide to go that route.
5) For me, my wife couldn't hide the fertile time. For one, I'm aware enough of her cycle and the fertility calendar. But also, the vagina feels different when you're about to ovulate. Sorry if this is a little much, but the viscosity of liquids feels different. Especially, if you're having plenty of regular sex, the guy can probably tell something's different.
My advice for the guy:
1) Brother, don't worry. This doesn't have any greater significance; it's not a reflection of you as a man or whatever negative thoughts are looping in your head right now. It's just a thing that happens to some guys.
2) Same as my advice for the woman, make sure you're having plenty of sex just for fun. Nothing repairs confidence like a good !**@! in the hay.
3) I'll share a tactic I used a few times. I didn't really have a problem getting an erection; I would just lose it too quickly. So, I decided to shorten the window of time an erection was needed. I would masturbate almost to the point of orgasm. Then my wife and I would have commence sex, and I would finish very quickly inside her. Admittedly, that's not the most romantic thing in the world or tons of fun for the lady, but sometimes when things aren't going exactly how you'd like, you have to make a sacrifice.
4) Turn off the voices in your head. Just shut out those all of those thoughts. Sex isn't about thinking. Just lie back and let your body do what it wants to do. Let the physical take over the mental.