Answer:

Most people tend to think of hair as something that just grows and grows. If not cut off, it eventually falls out at the root and just starts over again. But, like so many of life's surprises, when you stop and look at what's happening, it's a little more complicated than that.

Hair, or more accurately hair follicles, have three growth phases. The first is when the hair shaft is growing steadily. This may last up to several years for any particular hair. About 90% of the hairs on your head are in this growth phase at any one time. The second phase is called the resting phase. No growth occurs, and the hair just sits there. This phase may last up to several months. This is followed by the third phase wherein the cells of the hair follicle begin some intense activity in preparation for the production of another hair. Finally, a new hair starts to grow within the hair follicle, pushing out the old hair which ends up on your brush.

Abrupt, temporary, hair loss known by the wonderfully obscure but evocative name "telogen effluvium," may occur as a consequence of a number of unrelated but traumatic events in the body. These include surgery, an acute illness, severe stress, or, as in this case, pregnancy. In all of these cases, the event causes many of the hair shafts to enter the resting phase. I like to think of it as conserving energy for other more necessary activities. A couple of months later, when the new hair shafts begin to grow, the old ones fall out, and Jenny freaks.

As you can see, there is really nothing for her to worry about. By the time she is aware of her hair thinning, the re-growth process has already started. One answer we received mentioned the possibility of hypothyroidism. Hypothyroidism may occur in the few months after pregnancy, and it can also cause hair thinning. It's a very good thought, except that in Jenny's case the hair thinning would not be perceptible for another few months.