weight loss during pregnancy

At what point in a pregnancy do you start to worry about weight
loss? I know that the generic answer is eat to hunger and drink
to thirst and don't worry about it, just like it's feed a breast-
fed baby on demand. But at some point babies have failure to
thrive, and at some point weight loss must be worth worrying about.

I'm 5'2" and started this pregnancy at 170lb, which was
fairly overweight. I'm 11 (12?) weeks pregnant, and down to
159. I'm eating to hunger and then some, and drinking to thirst,
I get my vitamins every day, and I've got no morning
sickness to speak of. This week alone I've lost 2lb, and it's
not scale fluctuations. My diet is pretty varied, if heavy
on meats and fresh fruits and veggies.

While this is a great way to finish losing weight, when should
I worry and starting drinking carnation instant breakfast
milkshakes? I'm not worried about it yet, but I probably
will start to worry in another 5lb or so, especially if
the rate of loss continues like this.

Elaine the magical shrinking pregnant woman