#1 - Right side of the image seems a bit hot. The exposure difference in the background varies a lot - but mainly it's the shadow line near the bottom that bothers me. I don't think the person did you any favors wearing black - it's a lot harder to show the pregnancy thing with black I think. You're losing dimension because of the black shirt, and with that angle. Had it not been for the pose with her hands where they are I may not have been sure whether she was pregnant or not. Also blends into the pants pretty badly. I may have turned her a bit more to try to show a bit more profile on the stomach. It's probably a tough thing - she looks like she might normally have a little extra weight and then the baby weight could be making her more self-conscious.
#2 - I really dislike this one - they don't look like they even care for each other. I know you were trying to show her belly a bit more. I think you're balancing the light on your subjects pretty well, but the tree in the background and the sidewalk is too bright. I dig that door though it makes a nice background but it's a bit too involved in this shot. Much better in the first one.
#3 - Pretty good picture. Arms look a bit hot again. Again black not helping as much. Background light is much more controlled here and that really helps the image IMO.
#4 - Her expression is not good IMO. Also the color looks a bit off skin tone wise vs. the other shots. Not sure about the pose - maybe it's just that this couple isn't as good in front of the camera as others and there really isn't anything you can do about that except try to make them interact more and show some more emotion.
#5 - You're on to something here. You can see how the exposure disparity between her black dress and the bright background really gives you a good outline of her belly - that's great, but I wish there was just a bit more of the background brighter. Normally that could be distracting because it draws your focus - but in this case, the pregnancy is a focus and it would draw your attention there and that's good.
#6 - Pretty good standard maternity shot I think - skin is a bit hot perhaps.
Overall I think you did a good job and they should be happy with them. You mentioned that posing is something that you have trouble with - I find that a lot of times I don't pose a lot of things. I ask my clients to just interact, talk to each other, pretend like I'm not there - and then I try to capture the emotions. This doesn't work with everyone though. Another thing to try is a prop or two to use that they can mess around with - I know it can be corny but some little shoes or a teddy bear or something - it certainly doesn't have to be in every shot or even be used for that matter - sometimes it can just open up a client a bit more.
You said that people really had to convince her to have pictures taken - that can be a problem. If your client isn't really interested it will show - and in your case I think it shows here. Not really your fault and nothing you can do about it other than to try to engage them, talk with them, joke around - try to win them over. It can work sometimes. Won't work all the time. The best pictures are with both parties (clients and photographer) are comfortable and want to make some great pictures.
Lighting wise I think you're getting some falloff which is on your client which you don't want. Looks like a tough situation, so if these were wedding shots where you couldn't stop and pose and whatnot they would be awesome. For planned stuff, I think in this situation two lights would be ideal. The back sides of your subjects and in some the lower half of them is getting some inconsistency there.