Other than Troi in The Child, do we ever see a pregnant crewmember of the Enterprise D? Of course we see Keiko pregnant, but she's a civilian. I'm trying to imagine a maternity uniform.
In ST:VOY, Samantha Wildman was pregnant (over multiple episodes). Admittedly this is not exactly Enterprise D, but as you wrote "I'm trying to imagine a maternity uniform", it might be helpful:
He's a description of the costume from Screen Used:
This costume was made by the production for use by Nancy Hower in the role of the pregnant xenobiologist "Ensign Samantha Wildman" in the episode “Deadlock”. The costume is a standard type-B Starfleet uniform altered for maternity; long-sleeved, full-length, jumpsuit that has the usual gray undershirt attached to black jumpsuit pants with stirrups for the feet, a long-sleeved, pleated, black Starfleet tunic, Sciences Blue division strip, a Velcro patch on the left breast for a comm badge (not included), Velcro cuff adjustment, a zipper down the front, is open at the bottom. A sewn in tag reads "Voyager, Nancy Hower". Minor wear from use is present and two very small holes are located above the left chest and a small hole on the side of the right leg above the ankle.
You can see more detailed photos of the uniform here, but they're watermarked. The maternity alternation makes the jumpsuit look like separate top and pants because of added loose fabric that drapes over the abdomen. Apparently, you can also buy the actual costume for $1,249.