Wow, those sound like neat studies!
We were invited to participate in study of normal fetal growth and development, but, sadly, I turned them down. It turned out that they were going to do half hour ultrasounds (2-d and 3-d) every month (!!) for the duration of the pregnancy to measure growth. Having gone through a miscarriage (missed m/c at 12w last August), both DH and I are feeling pretty risk averse. No one *really* knows the long-term effects of excessive ultrasound exposure in utero, especially half-hour sessions monthly. I mean, unless you have a major complication and need such monitoring, what normal pregnant woman would have such scans done? Anyhow, I'm just not willing to take the risk.
Your studies, by the way, sound super-safe and not crazy like the one I was invited into. When DS was 1, we participated in some development studies, and they were pretty fun. We even have the video of his responses (he was looking at different objects of various colors, and they were apparently trying to elicit a 'surprise' response), and it's definitely cute to watch.