About 30 percent of drug-addicted individuals are women, and most of these are of childbearing age, according to laboratory test selection service ArupConsult. Exposure to marijuana in the womb may lead to delayed fetal growth and possible premature birth, the March of Dimes says. Babies exposed to prenatal marijuana may exhibit "withdrawal" type symptoms, such as excessive crying and shaking. Lab tests for marijuana exposure in newborns are available if the physician suspects maternal marijuana use.
Observe signs of suspected drug exposure. The doctor will determine that an infant or maternal drug test is warranted if she suspects drug use in the mother, or notes obstetrical signs of substance exposure in the baby, according to Arup Consult. These signs may include irritability, shakiness or prolonged distress. The doctor will then order lab tests of either the infant, the mother or both.
Collect a meconium sample. Meconium is the earliest infant stool, and begins to form in the womb between weeks 12 and 16 of pregnancy, according to ArupConsult. Meconium testing is one of the more commonly used approaches for drug screenings in infants. The doctor will collect a sample of the baby's meconium, and send it to a lab to be screened for various drugs. Among the drugs that meconium screening can detect are marijuana, cocaine, opiates, amphetamines and barbiturates. Meconium screening can detect drug exposure in the last four to five months of pregnancy.
Obtain a urine sample. In some cases, doctors order urine screenings to test for newborn marijuana exposure. However, according to ArupConsult, urine screenings are generally considered less effective, because they only can detect drug exposure over the past 10 days. In addition, urine is generally more difficult to collect in a newborn than a meconium sample.
Test the mother for drug use. According to the National Institutes of Health, health care professionals sometimes screen for maternal drug use through urine, although this approach only indicates drug use a few days before delivery. Doctors can also monitor suspected drug use in pregnant women through samples of sweat, saliva and hair.