Most plastic baby bottles are made with polycarbonate, a rigid durable plastic that has the potential to leach bisphenol A into your baby's drinks. Bisphenol A (BPA) has been linked to a number of health problems, including hormone disruption and obesity, and the National Toxicology Program recently concluded that there is "some concern for neural and behavioral effects" in infants and children, given current exposure rates to BPA.
Glass and other plastics are alternatives to baby bottles made with polycarbonate. Glass baby bottles don't leach toxic chemicals. Glass bottles are, of course, breakable, and put children at risk of injury in the case of an accident.
Additionally, glass can chip or crack when sterilized, which could lead to glass splinters in baby's beverage. As with plastic, careful and regular inspections of the bottle allow parents to detect any flaws in the glass. Recycle any scratched, cracked, or chipped glass bottle.
Bottles made of Number 2 HDPE, Number 4 LDPE and Number 5 PP Plastics: The compounds used to make these opaque bottles are not known to leach carcinogens or endocrine disruptors. Look on the bottom of the bottle for one of these recycling symbol: Number 2 (high-density polyethylene, HDPE), or Number 4 in the chasing-arrow triangle (low-density polyethylene, LDPE), or Number 5, (polypropylene, PP.)