A baby bottle is a bottle with a teat (also called a nipple in the US) to drink directly from. It is typically used by infants and young children when a mother does not breastfeed, or if someone cannot (as conveniently) drink from a cup, for feeding oneself or being fed.
Dimensions and design 
A large-sized bottle typically holds 270 ml; the small size 150 ml. It is composed of a bottle itself, a teat, a ring to seal the teat to the bottle, a cap to cover the teat and optionally a disposable liner.
The height-to-width ratio of bottles is high (relative to adult cups) because it is needed to ensure the contents flood the teat when used at normal angles; otherwise the baby will drink air. However, if the bottle is too tall, it easily tips. There are asymmetric bottles that ensure the contents flood the teat if the bottle is held at a certain direction.
Teats (or nipples) 
The teat itself is generally designed to be slimmer than the mother's nipple. Specialized teats are marketed that report attempting to mimic the shape of the breast to help babies to switch back and forth between bottle feeding and breastfeeding for cases where "nipple confusion" occurs.
Teats come in a selection of flow rates, marketed to be based on the age of the infant. Different flow rate teats either have more holes or larger holes. Variable flow rate teats are available for older infants. The hole is asymmetric so that by turning the bottle/teat, different flows can occur. Specialized teats are available for infants with cleft palate.
Vented bottles 
"Vented" bottles allow air to enter the bottle while the baby is drinking without the need to break the baby's suction during feeding. Alternatively a bottle liner can be used to enclose the formula instead of directly in the bottle. The liner collapses as the formula is drained.
Vented bottles work by allowing air to enter while preventing the liquid inside from escaping. Avent is a popular brand in this category in the United States. It works by an "anti-vacuum skirt" in the base of the teat, where it forms a seal with the bottle. The skirt acts as a one way valve, allowing air to enter the bottle but not liquids to leave. If the sealing ring is tightened too much, the skirt is compressed too tightly to allow it to open and the bottle will not vent. If the sealing ring is too loose, liquid leaks from the bottle.
There are multiple patents for technologies in this area. Initial designs called for a complex spring and valve system that was impossible to clean and sterilize. Current research is in specialized materials with microscopic pores that allow the entry of air without the escape of liquids. This avoids the caregiver having to get the sealing ring tension just right. It remains to be seen whether these materials can withstand the rigours of daily cleaning and sterilization. Another competitor, Dr. Brown's, offers a system whereby the vented air is conducted through a tube to the bottom of the bottle where the airspace is when the bottle is in use. This avoids the vented air from bubbling through the liquid and unnecessarily aerating the liquid. The aeration may cause nutrients in "human milk and infant formula (to) decrease in concentration . . . to a level that may be clinically significant".
Variations and accessories 
Bottles may be designed to attach directly to a breast pump for a complete "feeding system" that maximizes the reuse of the components. Such systems include a variety of drinking spouts for when the child is older. This converts the bottle into a sippy cup, a cup with lid and spout for toddlers, which is intermediate between a baby bottle and an open top cup. Bottles that are part of a feeding system may include handles that can be attached. The ring and teat may be replaced by a storage lid.
Accessories for bottles include cleaning brushes and drying racks. Brushes may be specially designed for a specific manufacturer's bottles and teats. Bottle warmers warm previously made and refrigerated formula. Coolers designed to fit a specific manufacturer's bottles are available to keep refrigerated formula cold. Special formula powder containers are available to store pre-measured amounts of formula so that caregivers can pre-fill bottles with sterile water and mix in the powder easily. The containers are typically designed to stack together so that multiple pre-measured amounts of formula powder may be transported as a unit.
Specialty, "designer" bottles are now quite common as novelty gifts for parents or just something interesting for the child. They either have special logos or are of special shapes (e.g., animals). Depending on the shape, these bottles can be quite difficult to clean. Another specialty bottle is made from heat sensitive materials that act as a built-in thermometer. If the contents are too hot, the bottle changes color.
Institutions can purchase ready-to-feed formula in containers that can be used as baby bottles. The lid screws off and is replaced by a disposable teat when the formula is ready to be used. This avoids storing the formula with the teat and possibly clogging the teat holes when formula is splashed within the bottle and dries.
If necessary, bottles can be sterilized by boiling in hot water, in a specialized bottle sterilization appliance (which typically uses steam) or in a specialized sterilization container that is microwaved. Modern bottles are difficult to sterilize in boiling water because they tend to float. Bottles were originally composed of glass which was dangerous when babies learned to feed themselves and held the bottle. Mainly for cost reasons, modern bottles are unbreakable plastic. Since bottles have to be made to withstand the heat of sterilization, the bottle can also withstand the heat of dishwashers.
There is some concern about Bisphenol A (BPA) leakage on polycarbonate bottles due to extended dishwasher or boiling. While bottles were traditionally sterilized in the past, unless there are infant health concerns, or concerns about water contamination, the current recommendation is that baby bottle sterilization can be replaced by cleaning with hot soapy water.
While infant formula is highly regulated in many countries, baby bottles are not. Only the materials of the teat and bottle itself are specifically regulated in some countries (e.g. British Standards BS 7368:1990 "Specification for babies' elastomeric feeding bottle teats"). In the USA, the Food and Drug Administration (FDA) also regulates teats and the bottle materials. In 1985 it tightened allowable levels of nitrosamines released from bottle teats. A 1999 Consumer Reports study suggested that some polycarbonate bottles release unsafe amounts of Bisphenol A; however industry critics contented the study demanded unreasonable conditions to which the bottles were subject. Findings since, however, have renewed the initial concerns (see Bisphenol A - Possible Health Risks).
Bottles with hard spouts go back to prehistory. The first consisted of urns with two openings: one for pouring the liquid into the bottle and the other to be put in the baby's mouth. Soft teats of various materials were tried but were very difficult to clean. The invention of vulcanized rubber provided a material that was soft and could withstand the heat of sterilization. Elijah Pratt of New York patented the first rubber teat in 1845. It took until the 1900s before the technology was improved for a soft teat that could be considered as practical for use by mothers who were not breastfeeding.
In the UK in 1999, "the feeding and sterilising equipment sector ... stands at £49m. Sales of feeding bottles account for 39% of the market", or £19.1m.
Baby bottles are discouraged by the World Health Organization, which considers exclusive breastfeeding followed by introduction of complementary foods with clean utensils safer. The American Academy of Pediatrics contends that feeding anything (even breast milk) to a child with a bottle can interfere with successful establishment of breastfeeding in the first two months. The transmission of some viral diseases through breastfeeding can be prevented by expressing breast milk and subjecting it to Holder pasteurization.
See also 
- Baby Friendly Hospital Initiative
- Breastfeeding promotion
- Haberman Feeder
- Infant formula
- International Code of Marketing of Breast-milk Substitutes
- List of bottle types, brands and companies
- Should You Sterilize Your Baby's Bottles?
- Glass Baby Bottles using Silicone Encapsulated Sleeve to prevent broken glass accidents.
- Kay, Jane (24 June 2011). "SAN FRANCISCO / Lawmaker wants state to follow city's lead with 'toxic toy' ban / Bill would bar certain chemicals in products". The San Francisco Chronicle.
- "Scientists warn about plastic in baby bottles". Chicago Tribune. 3 August 2007.
- World Health Organization. Infant and Young Child Nutrition: Global strategy for infant and young child feeding. Geneva, 2003.
- Tully DB, Jones F, Tully MR (2001). "Donor milk: what's in it and what's not". J Hum Lact 17 (2): 152–5. doi:10.1177/089033440101700212. PMID 11847831.