or vaginal sex," but that "measuring the exact risk of HIV transmission as a result of oral sex is very difficult" and that this is "because most sexually active individuals practice oral sex in addition to other forms of sex, such as vaginal or anal sex, when transmission occurs, it is difficult to determine whether it occurred as a result of oral sex or other more risky sexual activities". They added that "several co-factors may increase the risk of HIV transmission through oral sex"; this includes ulcers, bleeding gums, genital sores, and the presence of other STIs. In 2005, the World Health Organization estimated that 123 million women become pregnant worldwide each year, and around 87 million of those pregnancies or 70.7% are unintentional. Approximately 46 million pregnancies per year reportedly end in induced abortion. Approximately 6 million U.S. women become pregnant per year. Out of known pregnancies, two-thirds result in live births and roughly 25% in abortions; the remainder end in miscarriage. However, many more women become pregnant and miscarry without even realizing it, instead mistaking the miscarriage for an unusually heavy menstruation. The U.S. teenage pregnancy rate fell by 27 percent between 1990 and 2000, from 116.3 pregnancies per 1,000 girls aged 15–19 to 84.5. This data includes live births, abortions, and fetal losses. Almost 1 million American teenage women, 10% of all women aged 15–19 and 19% of those who report having had intercourse, become pregnant each year. Sexual activity can increase the expression of a gene transcription factor called ΔFosB (delta FosB) in the brain's reward center; consequently excessively frequent engagement in sexual activity on a regular (daily) basis can lead to the overexpression of ΔFosB, inducing an addiction to sexual activity. Sexual addiction or hypersexuality is often considered an impulse control disorder or a behavioral addiction. It has been linked to atypical levels of dopamine, a neurotransmitter. This behavior is characterized by a fixation on sexual intercourse and disinhibition. It was proposed that this 'addictive behavior' be classified in DSM-5 as an impulsive–compulsive behavioral disorder. Addiction to sexual intercourse is thought to be genetically linked. Those having an addiction to sexual intercourse have a higher response to visual sexual cues in the brain. Those seeking treatment will typically see a physician for pharmacological management and therapy. One form of hypersexuality is Kleine–Levin syndrome. It is manifested by hypersomnia and hypersexuality and remains relatively rare. Sexual