stigmatized. teachers who knew that specific children had been exposed to crack in utero may have expected these children to be disruptive and developmentally delayed. children who were exposed to cocaine might be teased by others who knew of the exposure, and problems these children had might be misdiagnosed by doctors or others as resulting from pce when they may really have been due to factors like illness or abuse. the social stigma of the drug also complicated studies of pce; researchers labored under the awareness that their findings would have political implications. in addition, the perceived hopelessness of 'crack babies' may have caused researchers to ignore possibilities for early intervention that could have helped them. the social stigma may turn out to be a self-fulfilling prophecy. research confounding factors a number of the effects that had been thought after early studies to be attributable to prenatal exposure to cocaine are actually due partially or wholly to other factors, such as exposure to other substances (including tobacco, alcohol, or marijuana) or to the environment in which the child is raised. pce is very difficult to study because of a variety of factors that may confound the results: pre- and postnatal care may be poor; the pregnant mother and child may be malnourished; the amount of cocaine a mother takes can vary; she may take a variety of drugs during pregnancy in addition to cocaine; measurements for detecting deficits may not be sensitive enough; and results that are found may only last a short time. studies differ in how they define heavy or light cocaine use during pregnancy, and the time period of exposure during pregnancy on which they focus (e.g. first, second, or third trimester. drug use by mothers puts children at high risk for exposure to toxic or otherwise dangerous environments, and pce does not present much risk beyond these risk factors. pce is clustered with other risk factors to the child, such as physical abuse and neglect, domestic violence, and prenatal exposure to other substances. such environmental factors are known to adversely affect children in the same areas being studied with respect to pce. most women who use cocaine while pregnant use other drugs too; one study found that 93% of those who use cocaine or opiates also use tobacco, marijuana, or alcohol. when researchers control for use of other drugs, many of the seeming effects of cocaine on head size, birth weight, apgar scores, and prematurity disappear. addiction to any substance, including crack, may be a risk factor for child abuse or neglect. crack addiction, like other addictions, distracts parents from the child and leads to inattentive parenting. mothers who continue to use drugs once their babies are born have trouble forming the normal parental bonds, more often interacting with their babies with a detached, unenthusiastic, flat demeanor. conversely, low-stress environments and responsive caregiving may provide a protective effect on the child's brain, potentially compensating for negative effects of pce. many drug users do not get prenatal care, for a variety of reasons including that they may not know they are pregnant. many crack addicts get no medical care at all and have extremely poor diets, and children who live around crack smoking are at risk of inhaling secondary smoke. cocaine using mothers also have a higher rate of sexually transmitted infections such as hiv and hepatitis. in some cases, it is not clear whether direct results of pce lead to behavioral problems, or whether environmental factors are at fault. for example, children who have caregiver instability may have more behavioral problems as a result, or it may be that behavioral problems manifested by pce children lead to greater turnover in caregivers. other factors that make studying pce difficult include unwillingness of mothers to tell the truth about drug history, uncertainty of dosages of street drugs and high rates of attrition (loss of participants) from studies. animal models one way to address problems with uncertainty about cocaine's effects due to confounding factors is to use animal models; these allow experimenters to study the effects at specific doses and times. studies have used mice, other rodents, rabbits, and primates. however, differences between species' physiology and gestation times mean findings in animals may not apply to humans. mice, rats, and rabbits have shorter gestational times, so experimenters must continue giving drugs after they are born to more closely model human gestation; however this introduces more differences. animals and humans metabolize drugs at different rates, and drugs that are highly teratogenic in animals may not be in humans and vice versa. animals cannot be used to measure differences in abilities such as reasoning that are only found in humans. animal studies in various species have found that cocaine impacts brain structure, function, and chemistry, and causes long-term changes at the molecular, cellular, and behavioral levels. animal model studies have shown that cocaine has the ability to cross the placenta and the blood brain barrier in the body. this is yet another example of the damage that can be done that can impact and effect the brain and body function and health overall. while the animal model is not as reliable for certain tests because we function differently, this test in particular gives us the idea of the level of damage that can be cause to the fetus of a pregnant women using cocaine during her pregnancy. in research studies on pregnant rats, injected cocaine did less damage to cells than injected nicotine, and more recovery occurred between doses. adult rats that were exposed to cocaine prenatally have deficits in learning, memory, and motor skills, and may have abnormalities in dopamine processing. animal research has also shown that offspring of males that used cocaine while their sperm were forming may go on to have abnormalities later in life.