Some newborns are small and immature. With modern hospital care, tiny infants usually survive, but it would be better for everyone—mother, father, baby, and society—if all fetuses developed for at least 35 weeks and all newborns weighed more than 2,500 grams (5½ pounds). (Usually, this text gives pounds before grams, but hospitals worldwide report birthweight using the metric system, so grams precede pounds and ounces here.)

The World Health Organization defines low birthweight (LBW) as under 2,500 grams. LBW babies are further grouped into very low birthweight (VLBW), under 1,500 grams (3 pounds, 5 ounces), and extremely low birthweight (ELBW), under 1,000 grams (2 pounds, 3 ounces). It is possible for a newborn to weigh as little as 500 grams, and they are the most vulnerable—about half of them die even with excellent care (Lau et al., 2013).

Remember that fetal weight normally doubles in the last trimester of pregnancy, with 900 grams (about 2 pounds) of that gain occurring in the final three weeks. Thus, a baby born preterm (three or more weeks early; no longer called premature) is usually, but not always, LBW. In addition, some fetuses gain weight slowly throughout pregnancy and are small-for-dates, or small for gestational age (SGA). A full-term baby weighing only 2,600 grams and a 30-week-old fetus weighing only 1,000 grams are both SGA, even though the first is not technically low birthweight.

Maternal or fetal illness might cause SGA, but maternal drug use is a more common cause. Every psychoactive drug slows fetal growth, with tobacco implicated in 25 percent of all low-birthweight births worldwide.

Another common reason for slow fetal growth as well as preterm birth is malnutrition. Women who begin pregnancy underweight, who eat poorly during pregnancy, or who gain less than 3 pounds (1.3 kilograms) per month in the last six months more often have underweight infants.

Unfortunately, many risk factors—underweight, undereating, underage, and smoking—tend to occur together. To make it worse, many such mothers live in poor neighborhoods, where pollution is high—another risk factor for low birthweight (Stieb et al., 2012).

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The causes of low birthweight just mentioned rightly focus on the pregnant woman. However, fathers—and grandmothers, neighbors, and communities—are often crucial. As an editorial in a journal for obstetricians explains: “Fathers’ attitudes regarding the pregnancy, fathers’ behaviors during the prenatal period, and the relationship between fathers and mothers … may indirectly influence risk for adverse birth outcomes” (Misra et al., 2010, p. 99).

As already explained in Chapter 1, each person is embedded in a social network. Since the future mother’s behavior impacts the fetus, everyone who affects her also affects the fetus. She may be stressed because of her boss, her mother, her mother-in-law, and especially her partner. One example is that unintended pregnancies increase the incidence of low birthweight (Shah et al., 2011).

Not only fathers but also the entire social network and culture are crucial (Lewallen, 2011). This is most apparent in what is called the immigrant paradox. Many immigrants have difficulty getting education and well-paid jobs; their socioeconomic status is low. Low SES correlates with low birthweight. Thus, newborns born to immigrants are expected to be underweight. But, paradoxically, they are generally healthier in every way, including birthweight, than newborns of U.S.-born women of the same gene pool (Garc’a Coll & Marks, 2012).

This paradox was first called the Hispanic paradox, because, although U.S. residents born in Mexico or South America average lower SES than people of Hispanic descent born in the United States, their newborns have fewer problems. The same discrepancy has now been demonstrated for immigrants from the Caribbean, Africa, Eastern Europe, and Asia. Why? The crucial factor may be fathers and grandmothers, who keep pregnant immigrant women drug-free and healthy, counteracting the stress of poverty (Luecken et al., 2013).

You have already read that life itself is uncertain for the smallest newborns. Ranking worse than most developed nations—and just behind Cuba and Croatia—the infant mortality rate (death in the first year) of the United States is 34th in the world, about 6 deaths per 1,000 live births, according to the United Nations. The main reason is that the United States has more extremely low-birthweight newborns (Lau et al., 2013).

For survivors born very low birthweight, every developmental milestone—smiling, holding a bottle, walking, talking—is later. Low-birthweight babies experience cognitive difficulties as well as visual and hearing impairments. High-risk newborns become infants and children who cry more, pay attention less, disobey, and experience language delays (Aarnoudse-Moens et al., 2009; Stolt et al., 2014).

Longitudinal research from many nations finds that children who were at the extremes of SGA or preterm have many neurological problems in middle childhood, including smaller brain volume, lower IQs, and behavioral difficulties (Clark et al., 2013; Hutchinson et al., 2013; van Soelen et al., 2010). Even in adulthood, risks persist: Adults who were LBW are more likely to develop diabetes and heart disease, partly because they are more often obese.

Longitudinal data provide both hope and caution. Remember that risk analysis gives probabilities, not certainties—averages are not true in every case. By age 4, some ELBW infants are normal in brain and body.

In some northern European nations, only 4 percent of newborns weigh under 2,500 grams; in several South Asian nations, including India, Pakistan, and the Philippines, more than 20 percent do. Worldwide, far fewer low-birthweight babies are born than two decades ago; as a result, neonatal deaths have been reduced by one-third (Rajaratnam et al., 2010).

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Some nations, China and Bangladesh among them, have improved markedly. In 1970, an estimated half of Chinese newborns were LBW; recent estimates put that number at 6 percent (Chen et al., 2013). In 1990, half of newborns in Bangladesh were LBW (World Bank, 1994, p. 214); now the rate is 22 percent (UNICEF, 2014). In some nations, community health programs aid the growth of the fetus. That has an effect, according to a study provocatively titled Low birth weight outcomes: Why better in Cuba than Alabama? (Neggers & Crowe, 2013).

In some nations, notably in sub-Saharan Africa, the LBW rate is rising because global warming, HIV, food shortages, wars, and other problems affect pregnancy. Another nation with a troubling rate of LBW is the United States, where the rate fell throughout most of the twentieth century, reaching a low of 7.0 percent in 1990. But then it rose again, with the 2012 rate at 7.99 percent, ranging from under 6 percent in Alaska to over 12 percent in Mississippi. The U.S. rate is higher than that of virtually every other developed nation (see Figure 4.7 for a sampling).

Many scientists have developed hypotheses to explain the U.S. rates. One logical possibility is assisted reproduction, since ART often leads to low-birthweight twins and triplets. However, LBW rates rose for naturally conceived babies as well, so ART cannot be the only explanation.

Added to the puzzle is the fact that several changes in maternal ethnicity, age, and health since 1990 should have decreased LBW, not increased it. For example, African Americans have LBW newborns much more often than the national average (almost 13.2 percent compared with 8 percent), and younger teenagers have smaller babies than do women in their 20s. However, since 1990 the birth rate among both groups has decreased markedly, so average LBW should decrease, not increase. Furthermore, maternal obesity and diabetes have increased since 1990; both lead to heavier babies. Nonetheless, more underweight babies are born.

Something must be amiss. One possibility is nutrition. Nations with many small newborns are also nations where hunger is prevalent. In both Chile and China, LBW fell as nutrition improved.

As for the United States, the U.S. Department of Agriculture (2014) reports an increase in the rate of food insecurity (measured by skipped meals, use of food stamps, and outright hunger) between the first 7 years of the twenty-first century and the next 7, from about 11 percent to about 15 percent. Hunger directly affects LBW, and food insecurity also increases chronic illness, which itself correlates with LBW (Seligman & Schillinger, 2010). Food insecurity rates are higher among women in their prime reproductive years than among middle-aged women or men of any age, and highest of all among single women with children.

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Another possibility is drug use. For example, the rate of smoking and drinking among high school girls reached a low in 1992, then increased, then decreased. Most U.S. women giving birth in the first decade of the twenty-first century are in a cohort that experienced rising drug use when they were younger; they may still suffer the effects. If that is the reason, the recent decrease in drug use among teenagers will result in fewer LBW babies soon.

Another possible cause of LBW is pollution. Air pollution is decreasing in the United States. If pollution is the cause, fewer LBW babies will be born. On the other hand, many chemicals are more common in the maternal blood stream than they were. Looking at food, drug, pollution, and chemical trends in various nations will help developmentalists understand how to prevent LBW in the future.