i think that there is a very powerful bit of knowledge still in the collective conscious that women must gain weight while pregnant. it's not totally accurate, but the "gain x amount if you are underweight, gain y amount if you are a healthy weight, gain z amount if you are overweight and perhaps consider maintaining or losing if you are obese" really hasn't totally filtered through yet. and since the new message is much more complicated than the last one, it's probably not going to stick so much. especially when the other messages swirling around pregnancy are "Eat for two" and other somesuch.
and i don't know how much you pay attention to branding, but there are certain things that just don't work. i think it was gerber in the early 90s that decided to expand their brand with frozen dinners for adults. i can't remember if i am mixing examples, but i think they called the meals single man or something close to that. they didn't sell well because adults did not want to choose a baby food brand for dinner, further complicated by the meal reminding them they they were alone. some times the branding that you do on one side prohibits that brand from expanding. and with pregnancy's message being simplified into the single word "gain" and ww being simplified into "lose" the opposite messages don't really mesh well.
and weightwatchers pretty much has always relied on slightly fewer calories than you need in order to see better results. just like every other diet book out there. big results keep people coming back, right? while it's great for results and sales and that ilk, it's harder to base a nutritious diet off of. figure most plans are going to run you about 1000 cals. figure that if you've actually got some weight to lose you're probably going to be maintaining at at least 2000 cals a day, which is about double what they plan is giving you and you start seeing where it is hard to adjust the targets. an overweight adult body has a lot of wiggle room when it comes to what it can take in terms of calorie and nutrient deficiencies. a body being grown from cells has a lot more stringent needs and doesn't any sort of backlog to pull from. an adult has all the foundations and walls put up and roofed; losing weight is just a form of remodeling. growing a child is actually setting all that stuff up. and how would you feel if you walked into the house that you had just built and decorated and as the movers move in the last load, the builder comes over, hands you the key and casually mentions that they might have run out of nails about halfway through, but they used duct tape and when they ran out of that, they switched to scotch tape, but enjoy your new house anyways?
and the weight watchers plans also encourage you to be more active. as with weightloss, where you are in your pregnancy and any issues you might have are better addressed by your doctor than a generic program. women who are really active before they are pregnant can sometimes stay really active through most of their pregnancies. other times, medical issues put women on bedrest early on. while there is likely a set of guidelines for what to recommend, having a doctor do it is a better idea than a ww leader. and it may be that some doctors do recommend a modified ww program for loss or maintenance during pregnancy. but that's the doc using medical knowledge and how the patient is to cater to their needs. i'm fairly certain you don't need any medical training to start up a ww group. and if you did, membership costs would go up and availability of groups would go down [fewer qualified leaders].