While newborn photography is probably my favorite type of shoot, I honestly have to say that maternity photography is a very close second. There's just something so wonderfully magical about the "baby bump", knowing it is home to a tiny living person with an entire future ahead of them.
As such, it deserves some reverence in it's representation in a photograph. Now, that's not saying that it needs to be a solemn and serious thing, either. Many moms-to-be, especially those in the last few weeks, sometimes need a bit of laughter and cheer. Often, they feel like their growing, changing shape makes them less than beautiful. That's so far from the truth! A photographer can really turn things around for them, in this department. Pregnancy is a miracle and women are usually never more beautiful (at least that's my opinion).
Now, there are some unwritten guidelines a photographer should follow. Most are just common courtesy. You are there to capture a moment, to take photos, to showcase memories, yes. That doesn't give free reign to touching their bellies. You wouldn't just walk up to someone in a grocery line and put your hand on a woman's stomach, so you shouldn't take liberties here, either. If you feel you must, please, at least have the decency to ask (but I hope you don't).
Keep the shoot short. This can be facilitated if it's well thought out and actually planned, as much as possible. During a longer session, say, with multiple locations or walking more than a short distance, allow for breaks. These moms are carrying extra weight in front, which causes back, hip and pelvic pain. There is also usually leg swelling and sometimes leg and foot cramps and pain in the knees. If she's relaxed and having fun, the photos will come out looking radiant and natural. This isn't really possible if she's in agony. Along this same line, try to remember that most, especially further along, aren't yoga instructors or gymnasts. Exaggerated position changes can take their toll. Try to avoid a photo shoot that goes like this: "Stand up, sit down, stand up, lay down, lean over, stand up...". You get my point. Yes, variety is the spice of life but try to keep posing changes long enough to get the variety you need, then move on the the next pose and do the same. This will help keep her more comfortable and relaxed while still getting you the shots you need.
Do feel free to encourage other family members to get in on the action. Sometimes they bring another child or children, often leaving dad or relative to awkwardly stand to the side, trying to corral them. The kids want their mom, dad is fussing and getting overwhelmed, this makes mom feel rushed and nervous and this will ultimately defeat the purpose of trying to keep her relaxed...the photos will certainly reflect this tension. Even something as subtle as a single hand of a soon-to-be-brother-or sister on her belly can make an interesting shot while, at the same time, show mom that there's no reason to feel tense. Just be willing to be versatile and adapt to change. It might just result in an upsale. Kids usually have short attention spans so if mom didn't plan on having anyone else in the shot, it will probably only be a few before little Johnny or Jane run off to dad, yet again, and you can get back to the original plan.
Keep poses simple and natural. This is still a photo shoot and the same "portrait rules" apply. Watch for tension in the arms, hands and fingers. Watch for furrowing at the brows. Above all, communicate! Don't be afraid to ask if it's time for a break or to make a joke to keep her smiling and comfortable so her arms relax. You want natural and gentle, not something that looks like she's gripping her belly.
Lastly, discuss her thoughts and desired outcome before the shoot takes place. This will help you better plan things out and will be more likely to result in print sales.