Bottle nipples should be regularly
checked (at least every two to three months) for these signs of wear and tear
that necessitate disposal of them:

• Breast milk or formula pours
  out in a stream. Liquid should drip steadily and rather slowly out of the
  nipple. Once it starts to come rushing out, this indicates that the hole has
  become too large and the nipple should be replaced.

• Discoloration: a change in the
  color of the nipple could be a sign that the nipple is deteriorating.

• Thinning: when the nipple
  becomes thinner, this an early sign that the nipple is weakening. A
  nipple's strength can be determined by pulling hard on the bulb: a nipple that
  is still in good condition should rebound into its original shape. If it
  doesn't, throw it away.

• Stickiness or swelling: this
  also means that the nipple is deteriorating.

• Cracks, tears, or breaks:
  when pieces of the nipple break off, they may lead to choking and thus
  must be disposed of.

You should replace
your baby's bottle if you notice cracks, chips, or breaks: these
can easily lead to cuts, pinches, or other injuries to your baby. This is
especially dangerous if you use glass bottles.