Each day, science and medicine add more to what is known about prematurity. Because of this, today's fragile babies have a greater chance of survival than those of 10 years ago. Here's the story so far.
When Is a Baby Considered Premature?
How Common Is Prematurity?
Calculating a Premature Baby's Age
Small, Medium, and Large
What Are the Causes of Prematurity?
Myths About Preterm Birth
When Is a Baby Considered Premature?
When a baby is born at 37 weeks or before, the infant is considered premature and the birth preterm. The calculation is based on the date of mother's last period and an evaluation of the physical and neurological maturity of the infant, using ultrasound measurements while the infant is still in the uterus and direct examination after birth. (Infants are post-term when they are in the womb for more than 42 weeks; this comes with its own set of concerns.) The limit of viabilitya baby's chance of survival outside the wombis about 22 weeks in utero, but as more is learned about such tiny babies and how to care for them, this lower limit may drop further.
Today about 10 to 12 percent of infants are born at or before 37 weeks' gestation. The percentage has been rising steadily in the United States since 1980 and is related to improved fetal survival, older maternal age, and the use of fertility drugs (and the subsequent rise in multiple births), among many other factors. The prenatal loss of fetuses through miscarriage or fetal death has shown a parallel decline over the same period.
The percentage of babies born with low birth weightconsidered to be less than five pounds or 2,500 gramsis up slightly since 1980 and is now at just under 8 percent. These percentages vary by race. For example, among African-American babies, the number with low birth weight or very low birth weight (under 3 1/3 pounds or 1,500 grams) is about twice as high as it is in other racial groups.
A premature infant's gestational age is the number of weeks completed in the womb at birth, as calculated from Mom's menstrual dates and the infant's maturational features at birth. The chronological age is the actual time since birth. Adjusted age is the chronological age corrected for the amount of prematurity.
For example, if a child was born three weeks ago (meaning his chronological age is 3 weeks) after spending 30 weeks in the womb (in other words, at birth his gestational age was 30 weeks), he'd be considered 33 weeks old, his adjusted age until he reaches term, which is 40 weeks.
At 6 months of chronological age, Jesse would have an adjusted age of 3 1/2 months: 6 months minus 2 1/2 months (10 weeks) early equals 3 1/2 months. Adjusted age is commonly used when the subject is the baby's development, but chronological age is best for calculating the timing of health care visits and immunizations.
An infant's relative size for a given gestational age is another way to characterize a newborn in addition to noting birth weight. An infant's weight relative to his gestational age is compared to well-established norms for growth in the uterus. An infant who is in the average range for his gestational age is termed "appropriate for gestational age" or AGA. If the infant's weight is low for his gestational age, he is termed "small for gestational age" or SGA. He might also be described as having "intrauterine growth retardation" or "IUGR." An infant whose weight falls above the average range is labeled "LGA" or "large for gestational age." Additional concerns come up with SGA and LGA infants, requiring further testing and monitoring. It's good for the baby to be AGA, even if he is very early and very small.
This means that there are three possible ways to describe infants of the same birth weight. For example, a five-pound infant may be an SGA baby at 42 weeks, an LGA baby of 30 weeks or an AGA baby if he is 36 weeks in gestational age. Depending upon which the five-pounder is, he will need specific kinds of diagnostic testing, different types of monitoring and support, and various kinds of follow-up recommendations.
There are many causes of prematurity; some are known and some are not. Known risk factors account for about half of the cases of preterm delivery. Known risk factors include:
Urinary tract infections, respiratory illnesses, and vaginal infections are known to be associated with preterm birth. Recently, gum disease and undetected viral illnesses have been associated with increased rates of prematurity. Even if there is no known infection at the time of a preterm birth, the placenta may show signs of infection.
Group B streptococcus bacteria in particular are linked to preterm birth even without causing any disease in the mother. That's why there's a test for that infection, using cultures or rapid screening on swabs of Mom's genital and rectal areas. If identified, this infection can be treated with antibiotics before or during labor to prevent the spread of infection to the infant. With the onset of preterm labor, infection is presumed to be a factor, so Mom and baby (after birth) are treated with antibiotics.
Soon we may have new ways of identifying and treating the several viral infections that many believe are the cause of a large number of preterm births that we now have no way to explain.
The membranes surrounding the fetus are a major barrier to infection. If these break or rupture early, the fetus is at risk for infection. This is called premature rupture of membranes or PROM. If any sign of infection occurs in the mother or is suggested in the monitoring of the fetus, an early delivery will be needed. Some early evidence suggests that PROM itself may be caused by infection.
Twins, triplets, and beyond are not often carried to term. Twins have a 25 to 50 percent chance of an early arrival, and the odds rise from there as the number of infants carried in the uterus increases. The uterus may get tight and begin contracting, the placental blood flow may decrease, or the placenta(s) may wear out. For whatever reason, multiples often come early or a decision is made to deliver them early.
Infants with irregularities in development may start knocking at the door early. Ultrasound testing often helps identify them. If the fetus needs an intervention before the due date, a premature delivery may be planned.
Moms with uterine or cervical abnormalities; chronic illnesses such as kidney disease, preeclampsia/eclampsia (a pregnancy-related illness with high blood pressure), or diabetes; or a poorly functioning, bleeding, or damaged placenta usually require early delivery of the baby. Delivery may be by Cesarean section (C-section) or induced for the well-being of the mother and/or the baby.
Other factors have been linked to a higher rate of preterm birth. These involve:
Most preterm moms and dads wonder what they did to cause an early delivery and feel some guilt. In the vast majority of cases, there is nothing that could have been done to prevent an early birth. But myths continue and include:
About 20 percent of preterm infants are delivered electively (have a planned delivery) because of the mother's or infant's condition. Many of these are delivered by C-section to avoid the stress of labor; some births have labor induced. Another 30 percent have a preterm delivery after the membranes rupture, and about half of all preterm births are delivered after the start of preterm labor.
Labor is started or an operative delivery proposed if the infant is in distress, termed fetal distress. Decreased fetal movement, poor response to induced uterine contractions (a stress test), diminished or arrested fetal growth, or placental bleeding or separations (abruption) are distress conditions and are reasons to consider an early delivery.
The chances of needing a Cesarean delivery rise as the number of infants carried increases to twins, triplets, and beyond. This is due in large part to the different positions these womb-mates take besides the usual head-down position.
If Mom has a high-risk condition and/or a very preterm delivery is imminent, she may be transferred to a high-risk center for delivery. This will allow her needs and those of her infantwho will require very specialized careto be met. Expect a cast of thousands (no, not really thousands) to be at the delivery to assure that the needs of the baby and mother are met completely and rapidly. It's better to be overly prepared than to need but not have essential equipment and personnel.
If an infant is found to be unexpectedly premature and/or develops complications of a variety of sorts, she may be taken by plane, helicopter, or ambulance to a regional perinatal center where her needs can be met. When her medical condition stabilizes, she is often transferred back to a hospital nearer home to continue to grow and mature until she's ready to go home. Although these transfers are disappointing to the family, the development of NICUs and perinatal treatment centers has been a major advance allowing small infants to survive and thrive in greater and greater numbers all the time.
Preventing all preterm births is not possible, but there are some things that will lower the numbers. These include good prenatal care and good medical care between pregnancies. Women with a high-risk condition or with any chronic or acute health condition should see their health care provider early and often. Good nutrition, appropriate weight gain, and no smoking or drug use also will reduce the preterm rate, as will better diagnosis of subtle infections.