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Late Preterm Birth

Preterm birth is defined as an infant born prior to 37 weeks gestation. The frequency of preterm birth is 1 of 8 infants born in the U.S. Late preterm birth has been defined as those occurring between 34 and 36 weeks of gestation. Research has shown that birth at or after 39 weeks allows the infant to develop to a maturity that allows the infant to best survive outside the womb. The World Health Organization (WHO) and the American College of Obstetricians and Gynecologist (ACOG) have established the definition of premature delivery as a delivery occurring prior to the 37th week of gestation or prior to the 259th day after the first day of the last menstrual period. Late preterm birth is defined as birth occurring between the 239th to 259th day of gestation. The WHO now ranks premature birth as being very early premature births prior to 31 weeks and six days, early premature birth between 32 weeks and 33 weeks and six days, and late premature birth between 34 weeks and 36 weeks and six days. Over the past two decades the number of premature births has increased with the largest portion of the increase occurring in the late premature group. In a study performed by the British Columbia Perinatal Database Registry, maternal risk factors such as infections, hypertension, diabetes and premature rupture of membranes were associated with increase of late preterm births. The increase was believed to be due to increased obstetrical care that detected maternal problems resulting in the necessity to deliver the infant early as the disorders would result in a potentially worse outcome if the pregnancy was allowed to continue.

Factors that have led to the rise in late premature births

  • As mentioned above, improved obstetrical care detects maternal and fetal conditions that could increase the need for delivery of the infant prior to week 37.
  • Stead increase in multiple births in the U.S. over the past 20 years, associated with an increase in women over 30 having pregnancies and the use of assisted reproductive technologies. Approximately 60 percent of twins tend to deliver around 35 weeks of gestation and triplets tend to deliver around 32 weeks.
  • Several studies have reported that single infants conceived through assisted reported technologies have a higher risk of premature delivery as compared to single pregnancy conceived naturally.
  • Women older than 30 have the highest risk of delivering prematurely due to increased risk of hypertension, diabetes and other complications of pregnancy. These women also tend to seek assistance with getting pregnant and have a higher risk of conceiving twins spontaneously.
  • Late entry to care and inaccurate dating of the pregnancy tend to result in medically assisted deliveries of late premature infants. Ultrasound done after 20 weeks is inaccurate for determining accurate due dates.
  • Overweight and obese females have a higher risk of preterm delivery, due to obesity-related medical disease such as hypertension and diabetes.

Risks to the infant of late preterm birth
Late preterm infants have a higher morbidity rate, more frequent hospital admissions during the first year of life and higher risk of developing neurological and developmental problems. In a recent study morbidity doubled for each week the infant was born prior to the week 38. This led to a longer hospital stay after birth, a 10-time increase in cost of care, and a five-time increase of cost for care in the first year of life. The following is a partial list of complications that can be seen in the late preterm infant:

  • Hypothermia – Low body temperature because the infants do not have as much body fat for insulation, is unable to generate heat efficiently and tends to lose heat through the skin easier.
  • Low blood sugars – In late preterm infants, low blood sugar tends to occur three times more frequently than in term infants.
  • Respiratory problems – These occur due to immature lung structure and immature breathing capacity. This can lead to rapid, labored breathing, pneumonia, respiratory failure and the need to be placed on a ventilator.
  • Increased bilirubin – This is due to immaturity of the liver leading to jaundice. If jaundice is untreated or becomes prolonged it can lead to damage of the brain. Jaundice is the most common cause of readmission to the hospital for the late preterm infant.
  • Difficulty feeding – Late preterm infants may have non-coordination of the oro-buccal reflex and poor swallowing mechanisms.

Late preterm infants may have long term problems including cerebral palsy, mental retardation; psychological, behavioral and emotional disorders; learning disabilities and risk for failure to thrive. Immaturity of the brain at birth plays a major role in the risk of development of these problems.

ACOG has recommended that no deliveries should be electively done prior to 39 weeks of gestation unless the mother’s or the infant’s health dictate the need for earlier delivery. If you have a history of a previous preterm delivery, discuss this with your physician.

More about Dr. Aulds

Donald G. Aulds, MD is an Obstetrician and Gynecologist and currently serves as the Medical Director for both the Women's Center and the Best Start Program of North Alabama. He is a Diplomat of the American Board of Obstetrics and Gynecology and Fellow of the American College of Obstetricians and Gynecologists.

Dr. Aulds completed his medical education at Louisiana State University School of Medicine, New Orleans, LA and his Internship and Residency in Obstetrics and Gynecology at Ochsner Medical Foundation, New Orleans, LA.

Dr. Aulds has been an active member of the Huntsville Hospital Medical Staff since 1980.