There's no doubt Hollywood is on the baby train and we're all going along for the ride. From who's expecting to how they lose the baby weight, we can't seem to get enough of the beauties with the bumps.

But when some starlets announce their baby news, we're left asking bigger questions than what Hollywood-esque name the parents will choose. Case in point: When Nicole Richie said she was expecting, some wondered how such an underweight woman could actually conceive, and more importantly, what challenges might an underweight woman (or one with a not-so-healthy history) face during pregnancy?

As it turns out, the challenges are numerous. The weight and health of the mother during pregnancy has an enormous impact on the health of the baby. Malnutrition before and around the time of conception prevents the placenta from developing fully. A poorly developed placenta can't do its job of delivering optimal nourishment to the fetus. The infant can be born too small and possibly have physical and even cognitive impairments.

Underweight women tend to give birth to underweight babies, especially if they don't gain the recommended amount of weight during pregnancy. And that's a serious problem. Low-birth weight babies (five-and-a-half pounds or less) are almost 40 times more likely to die during their first year and when they survive, they may have problems later on.

So how much weight should a woman plan to gain during pregnancy to increase the odds of having a healthy baby? The recommended weight gain for someone of normal weight (18.5 to 24.5 BMI) is 25 to 35 pounds. For someone who is underweight (less than 18.5 BMI), the recommended weight gain is 28 to 40 pounds. For someone who is overweight (25.1-29.9 BMI), a gain of 15 to 25 pounds is ideal. Obese women (those with a BMI greater than 30) are expected to gain 15 pounds.

The rate of the gain is also important and should be slow, steady and consistent. A woman of normal weight should gain about three to five pounds in the first trimester, and about one pound a week during the rest of her pregnancy.

In a culture (Hollywood in particular) that is obsessed with thinness, weight gain during pregnancy can be difficult. Yet, the mother's diet during pregnancy is clearly a critical factor affecting the health of the baby. The embryo is growing quickly and depends on a strong infusion of vitamins, minerals, essential fatty acids and protein. A lack of nutrients in the mother's diet could put her growing baby at an enormous disadvantage and adversely impact the baby's intelligence, future behavioral patterns and syndromes.

Aside from the mother's weight, her history of smoking, alcohol and drug use also impact a pregnancy, and in a major way. It's now well-known that the mother's stress levels during pregnancy can have a profound affect on the baby's temperament and well-being. Drug and alcohol use can result in birth defects, not to mention a baby that is high strung or nervous. Before the placenta is completely formed (approximately 12 weeks), alcohol and drugs diffuse directly into the tissues of the developing embryo and can impair the development of the fetus. And while smoking is never good, the damage it does is magnified during pregnancy. Smoking restricts the blood supply to the fetus limiting nutrient delivery, oxygen delivery and waste removal.

Underweight women, or those who use drugs and alcohol recreationally, do not generally make ideal candidates for pregnancy. However, they could certainly change their behavior patterns and go on to have robust pregnancies and happy, healthy children. For many people, pregnancy serves as a wake-up call that moves them away from self-destructive habits and toward a healthier lifestyle for themselves -- and their babies.