Unfortunately, that is not always the case. And it usually isn't necessarily that they have anything against breastfeeding, but instead, many Pediatricians and other health professionals just haven't received enough education or training to be supportive of breastfeeding, especially when problems occur. It is important to remember that the increase in breastfeeding rates and the availability of lactation consultants only began in the middle to late 70's, so doctors trained before and during this period may not have had much experience with breastfeeding or lactation support professionals.
Even doctors that have recently finished training may not have received formal education about breastfeeding. In my own Pediatric's residency, most of the time I spent taking care of newborns was in the Neonatal Intensive Care Unit with preemies and newborns that were very sick and I didn't have much exposure to breastfeeding mothers. Fortunately, one of the 'Ten Steps to Support Parents' Choice to Breastfeed Their Baby' of the American Academy of Pediatrics is to 'Train all physicians and office staff in skills necessary to support breastfeeding.'
So how do you find a doctor that is supportive of breastfeeding and that can help you have a positive breastfeeding experience? Lactation consultants in your area should be able to recommend Pediatricians that are known to be supportive of breastfeeding and steer you away from those that aren't.
Another good way is to just ask your doctor how he feels about breastfeeding. A prenatal visit or a 'new mom' consult is a good way to get to know a new Pediatrician or to just talk about your plans for breastfeeding your new baby. You can find out what will happen if you do have problems breastfeeding and ask for recommendations to help maximize the chance that you will breastfeed effectively, including breastfeeding as soon after the delivery as possible, avoiding supplementing with a bottle or using a pacifier, and rooming in with and feeding your newborn on demand.
In addition to finding a Pediatrician that is supportive of breastfeeding, you can maximize your chances of breastfeeding successfully by learning as much as you can about breastfeeding and potential problems that may come up. There are many excellent books about breastfeeding and you should consider reading while you are still pregnant, with one of my favorites being 'The Nursing Mother's Companion' by Kathleen Huggins.
Having a breastfeeding support system in place is also helpful. This should usually include a lactation consultant or lactation specialist. A great suggestion I once heard is to keep the phone number of your lactation consultant by the phone with your list of emergency numbers. This way you always have it handy and can get help when you need it. Family members and friends who have breastfed are other good sources of support.
You may also want to take a prenatal breastfeeding preparation class, or if you have any risk factors that make it more likely that you will have difficulty with breastfeeding, then schedule a prenatal evaluation with a lactation consultant and/or a prenatal breast examination.
So how do you know if you doctor isn't supportive of breastfeeding? A good way to tell is if at the first sign that you are having problems breastfeeding, your doctor recommends supplementing with a bottle, changing to formula or 'just keep trying'. While there are some situations where supplemental feedings are medically necessary, especially if the baby is dehydrated or has excessive weight loss, giving supplements, in addition to ensuring the health of the baby, should include a goal of fixing whatever is going wrong with breastfeeding, so that you will ultimately be able to exclusively breastfeed. This can include increasing the mother's supply of breastmilk and/or helping the baby latch on or suck properly. If you just supplement with a bottle when you are having problems, then there is a good chance that you will push your baby to wean early. And it is important to remember that supplements aren't usually necessary, and even when they are, there are alternatives to using a bottle, such as using a lactation aid, cup or finger feeding.