PROVIDENCE, R.I.--Premature infants born to women who suffer mild high blood pressure during pregnancy may not fare as badly as doctors think, according to a new study.
Hypertension in pregnancy is a known risk factor for prematurity, but most neonatologists do not distinguish between mothers with mild hypertension and mothers with severe hypertension. However, the new study shows that the incidence of respiratory distress syndrome, a common morbidity of low-birth-weight infants, is much lower in newborns of mothers with mild hypertension than in those of mothers with severe hypertension or mothers with no hypertension.
Although an expectant mother's high blood pressure may cause fetal stress and prematurity, the study provides evidence that mild maternal hypertension actually accelerates maturation of the lungs and nervous system in infants.
"Both obstetricians and neonatologists can use this information in anticipatory clinical management and patient counseling," said Dr. Betty Vohr, one of the study's authors. She is professor of pediatrics at the Brown University School of Medicine and director of neonatal follow-up at Women & Infants Hospital of Rhode Island.
"If mild hypertension in an expectant mother remains controlled, then the physician can be somewhat positive about the newborn's outcome," she said. "Severe hypertension, however, remains a serious complication for both mother and infant. High blood pressure in pregnancy is a high-risk complication monitored and managed by obstetricians. Our data suggests, however, that there appear to be some benefits for very-low-birth-weight infants from the stress imposed by mild hypertension."
The study appeared last July in the journal Archives of Pediatric and Adolescent Medicine. Its lead author is Dr. Chang-Ryuk Kim, Hanyang University School of Medicine, Seoul, South Korea. Kim was a visiting clinical fellow in neonatology at Women & Infants Hospital.
In the study, 39 very-low-birth-weight infants born to mothers with mild or severe hypertension were compared with a control group of 78 very-low-birth-weight infants born to mothers with normal blood pressure.
The researchers found that very-low-birth-weight infants born to women with both mild or severe high blood pressure were significantly smaller for the same date of delivery and up to 20 percent lighter than their counterparts born to mothers with normal blood pressure. However, the incidence of respiratory disease was significantly lower in newborns of mothers with mild hypertension than in those whose mothers had severe hypertension or no hypertension.
In fact, those in the mild group had fewer days of ventilator therapy and lower oxygen requirements than infants in either the severe group or the control group. The incidence of apnea, a temporary stoppage in breathing, was also significantly lower in the mild hypertension group than in the severe hypertension group.
The findings suggest that the severity of maternal high blood pressure determines whether it is beneficial or harmful to very-low-birth-weight infants, Vohr said. There is no absolute agreement in medical literature on the definition of maternal hypertension, but the definitions of preeclampsia and severe preeclampsia--toxic conditions during pregnancy marked by high blood pressure and other symptoms, which account for most maternal hypertension--are fairly well established, she said.
Preeclampsia is associated with 10 percent of all pre-term births of infants weighing less than 2.2 pounds. This study indicates that doctors need to monitor the severity of hypertension and adjust their treatments and expectations accordingly, Vohr said.
"Doctors may treat mild hypertension with a low-salt diet and medication," she said. "If the disease progresses to severe hypertension, this indicates that the risk to both the mother and fetus has multiplied and emergency delivery may be indicated."
Hypertension in its most severe form can lead to seizures and coma for the expectant mother and to severely retarded growth and possible death for her fetus. Because of these complications, most premature babies born to mothers with severe high blood pressure are delivered by Cesarean section.
Obstetricians managing a patient will be monitoring the severity of the hypertension on an ongoing basis, Vohr said. "Neonatologists need to review the pregnancy records of women with hypertension to facilitate optimal management of the newborn by differentiating between mild and severe forms of hypertension," she said.
"The bottom line is that although mothers with both mild and severe hypertension will have an increased incidence of infants who are small for the same date of delivery and premature, the infants born to mothers with mild hypertension will have an easier newborn hospital course with less respiratory distress syndrome, apnea, need for a ventilator, need for oxygen and chronic lung problems," Vohr said.
The study's other author is Dr. William Oh of Women & Infants Hospital, chairman of the Department of Pediatrics at the Brown University School of Medicine.