Trends in Preterm Birth

As many reports have indicated, the proportion of preterm births has risen fairly steadily since 1990 (Figure 1-1). Note, however, that some of the change in the rate of preterm birth is likely reflective of changes in the way in which gestational age is measured (see Chapter 2 for a discussion). In the early to mid-1990s the percentage of preterm births remained stable at about 11 percent. There was a slight decline from 11.8 percent in 1999 to 11.6 percent in 2000. In 2004, the percentage rose to its highest level since 1990, from 10.6 to 12.5 percent. Since 1981 (when the percentage was 9.4 percent), the proportion has increased more than 30 percent (CDC, 2005a).

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FIGURE 1-1 Preterm births as a percentage of live births in the United States, 1990 to 2004. SOURCES: CDC (2001, 2002, 2004a, 2005a).

Gestational Age

Changes in the distribution of preterm births by degree of prematurity have occurred over time. Although the group of infants with the greatest morbidity and mortality are those who are born at less than 32 weeks, infants born at between 32 and 36 weeks represent the greatest number of preterm births. Figure 1-2 illustrates that between 1990 and 2003, the percentage of preterm birth was the highest for infants born at between 32 and 36 weeks of gestation. Among singleton births, the 7 percent rise in preterm births between 1990 and 2002 was attributable to the birth of infants of these gestational ages (CDC, 2005a). In contrast, during the same time period the proportions of preterm births occurring at less than 32 weeks of gestation declined from 1.69 to 1.57 percent. (See Appendix B for further discussion of gestational age distribution and trends by race and ethnicity.)

10 8

1990 1995 2001 2000

FIGURE 1-2 Preterm births as a percentage of live births by gestational age. SOURCE: CDC (2002a).

Mortality Associated with Preterm Birth

The overall Infant Mortality Rate (IMR) decreased from 9.1 deaths per 1,000 live births in 1990 to 6.9 in 2000. An interruption in this steady decline was evidenced in 2002, with an increase in the IMR to 7.0 deaths from 6.8 deaths per 1,000 live births in 2001, the first rise since 1958 (CDC, 2003a, 2005e). Analyses assessing the relative contribution of the change in the distribution of births by gestational age and in gestational age-specific IMRs to the change in the IMR revealed that 61 percent of the rise in the IMR from 2001 to 2002 was due to changes in the distribution of births by gestational age. Thirty-nine percent of the increase was due to changes in gestational age-specific mortality rates (CDC, 2005c,d).

In 2002, the IMR for infants born at less than 37 completed weeks of gestation was 37.9 deaths per 1,000 live births, whereas the IMR was 2.5 deaths per 1,000 live births for infants born at between 37 and 41 weeks of gestation (full term). Figure 1-3 displays IMRs by gestational age from 1999 to 2002. In 2002, approximately 64 percent of all infant deaths were for the 12.1 percent of infants who were born at less than 37 weeks of gestation and 53.7 percent of all infant deaths were for the 2 percent of infants born at less than 32 weeks of gestation. (Also see Appendix B for data reflecting gestational age-specific IMRs and geographic variations in mortality rates.)

CD O

TO CD

440 390 340 290 240 190 140 90 40 -10

weeks 28-31 weeks

1999

2000

2001

2002

FIGURE 1-3 IMRs by gestational age, United States, 1999 to 2002. SOURCE: CDC (2005c).

Geographic Differences

The proportion of preterm births varies considerably across different regions of the country (Figure 1-4). This variation may be related to state demographics, such as the distribution of maternal ages, multiple births, and race and ethnicity (CDC, 2005b). In 2003, the lowest percentages were in the Northeast states (particularly New England), followed by the West, the Midwest, and the South. It is notable that within the western region of the country, where many states have proportions of preterm birth below the national average, Nevada's was 13 percent. Among southern states, Georgia and Virginia had the lowest percentages of preterm birth: 12.6 and 11.8 percent, respectively.

FIGURE 1-4 Percentage of preterm birth by U.S. state, 2003. SOURCE: MOD (2005d).

The trends within the four major geographic regions show that the proportion of preterm births in the Northeast increased throughout the 1990s, with a small decline from 1999 to 2000 (MOD, 2005). From 1996 to 2002, the proportion remained below the national percentage (12.1 percent). The percentage of preterm birth in the Midwest during this time period was roughly equivalent to the national percentage. Western states had the lowest proportion of preterm birth from 1996 to 2002 compared with the percentages in other parts of the country. Southern states had the highest percentage of preterm birth in the United States from 1996 to 2002. This percentage steadily increased and remained above the national average during this period. (See Appendix B for further discussion of the geographic and sociodemographic variations in the rates of preterm birth.)

International Comparisons

It is widely reported that the rates of preterm birth in the United States are anomalously high compared with those in other developed countries. However, there is a paucity of published international reports providing unbiased comparisons of the rates of preterm birth among coun tries. Many of the data that are available have compared countries by the rates of low birth weight. For example, the United Nations Children's Fund (UNICEF) and the World Health Organization (WHO) (UNICEF and WHO, 2004) reported estimates of the incidence of low birth weight. Nevertheless, such reports emphasize that international comparisons and trends must be interpreted cautiously and typically do not allow conjectures on the range of determinants that may underlie the observed differences in the rates of preterm birth to be made. Table 1-1 displays the incidence of low birth weight in selected developed and developing countries.

TABLE 1-1 UNICEF and WHO Estimates of the Incidence of Low Birth Weight_

No. of Low-Birth-% Low-Birth- Weight (1,000s) % of Births Not

TABLE 1-1 UNICEF and WHO Estimates of the Incidence of Low Birth Weight_

No. of Low-Birth-% Low-Birth- Weight (1,000s) % of Births Not

Country

Year

Weight Infants

Weighed

Australia

2000

7

16

NA

Canada

2000

6

19

NA

China

1998-1999

6

1,146

NA

Cuba

2001

6

8

NA

Denmark

2001

5

3

NA

Finland

2001

4

3

NA

France

1998

7

51

NA

Germany

1999

7

49

NA

Guatemala

1999

13

53

22

India

1999

30

NA

Ireland

1999

6

7,8373

71

Japan

2000

8

93

NA

Malaysia

1998

10

53

NA

Mexico

1999

9

212

NA

Norway

2000

5

3

NA

Russian Federation

2001

6

79

NA

South Africa

1998

15

155

32

Spain

1997

6

23

NA

Sudan

1999

31

335

NA

Sweden

1999

4

4

NA

Switzerland

1999

6

4

NA

United Kingdom

2000

8

52

NA

United States

2002

8

323

NA

SOURCE: UNICEF and WHO (2004). Note: NA = Not available.

Several cautions are noted in comparing these data across countries (UNICEF and WHO, 2004). First data from industrialized countries are obtained mostly from service-based data and national birth registration systems, while data from developing countries are derived from national household surveys and other routine reporting systems. In addition, the majority of infants in developing countries are not weighted at birth, although an attempt is made to adequately adjust the data. Among developed countries, there are differences in definitions used reporting births (for example, cut-offs for registering births and birth weight). This data is also not cor rected for variables such as maternal age, race, social and economic disadvantage, and health care factors.

A report by the Canadian Perinatal Surveillance System cited a rate of 7.1 preterm births per 100 live births in 1996; in comparison, the rate in Australia was 6.9 and that in the United States was 11.0 (McLaughlin et al., 1999). Reporting of data on preterm birth rates at the international level is problematic, however, because substantial differences in the definitions of live births and fetal deaths exist in different countries, which affects the rates of preterm birth. These differences stem from variations in vital record reporting, laws, and procedures. Among the different countries, the impact of different standards of practice related to the measurement of ges-tational age is often unclear (see Chapter 2 for discussion of measurement issues). It may not be known what measure is typically used to estimate gestational age, for example, ultrasound or the first day of the mother's last menstrual period. Some nations rely on periodic surveys rather than vital records to establish preterm birth rate trends. In addition to measurement issues, there are variations demographic and health variables, as described above. Therefore, observations of marked differences in preterm rates between European countries and the United States are fraught with the potential for error. Currently, the twofold and greater differences in very preterm rates among U.S. states are largely unexplained. Establishing which factors underlie similar or greater variations among countries is an even larger task.

Method of Delivery

There has been a shift toward earlier delivery for infants of all gestational ages, which may reflect the increase in use of practices such as induction of labor and cesarean delivery (Martin et al., 2005; MacDorman et al, 2002). In 2003, the rate of cesarean delivery was 27.5 percent of all births. The rate was 20.7 percent of all births in 1996 and 26.1 percent of all births in 2002. While the total rate of cesarean births increased for all gestational ages between 1996 and 2003, the highest increase was for preterm infants born at 32 to 36 weeks and those born full term (37 to 41 weeks gestation). In 2003, 49.5 percent of infants born at less than 32 weeks gestation and 37.3 percent of infants born at 32 to 36 weeks gestation were delivered by cesarean.

The rise in cesarean rates corresponds to a rise in maternal age. In 2003, the cesarean delivery rate for women 25 to 29 was 26.4 percent. For 35-39 year old women, the rate was 36.8 percent. This may be related to increased multiple births for older women, biological factors, or patient-practitioner concerns (Ecker et al., 2001; Martin et al., 2005). Some findings suggest relatively low risks for maternal morbidity (for example, anesthesia-related complications, infection) (Bloom et al., 2005; Lynch et al., 2003), while others suggest that there are increased risks such as re-admission in the postpartum, infection, and complications related to anesthesia, among others (Koroukian, 2004; Hager et al., 2004; Liu et al., 2005; Lumley, 2003).

Maternal Age

Dramatic demographic changes in the late 20th century have resulted in increased levels of education and rates of employment among women, including the employment of married women and mothers of young children. The U.S. Department of Labor (DOL, 2005) reported that in 2003 more than half of married American women and more than half of mothers of young children were employed, with the most highly educated women being the most likely to be working. The current proportion of women in the civilian workforce (56 percent) presents a 54 percent change from the proportion in 1980 (DOL, 2004). There has been an increasing trend for women to delay childbearing into their 30s. In 2003, women ages 30 to 34 experienced the highest birth rate for women in this age group since the mid-1970s and women ages 40 to 44 had the highest birth rate for women in this age group since the late 1960s (CDC, 2005a). For women ages 35 to 39, the birth rate increased 47 percent between 1990 and 2003, although the increase in the population of women aged 35 to 39 was only 7 percent (CDC, 2004a, 2005a) (Figure 1-5). A slight increase in the birth rate among women between the ages of 25 and 29 was observed in 2003. In contrast, over the past decade, the birth rates among adolescents (ages 15 to 19) and women ages 20 to 24 have decreased. The current birth rate among adolescents is the lowest rate recorded for this age group in the United States (CDC, 2005a).

140 120 100 80 60 40 20 0

FIGURE 1-5 Birth rates by age of mother, 1993 to 2003. The rates represent the number of live births per 1,000 women in each group. SOURCE: CDC (2005a).

Women age 35 and older have increased rates of preterm birth (see Figure 4-1 for discussion and also see Appendix B). Although adolescents also have increased rates of preterm delivery, the numbers of births among women in this age group, as noted above, have decreased in the last decade. Older mothers are more likely to have underlying medical conditions, such as diabetes and hypertension. The rates of these diagnoses reported in birth certificate data have risen since the early 1990s, in tandem with the rise in the mean maternal age (CDC, 2003c). These chronic health problems are associated with adverse birth outcomes, such as growth restriction, preeclampsia, and abruption, leading, in turn, to increases in the rates of indicated preterm deliveries.

ARTs, Multiple Births, and Preterm Birth

The incidence of multiple births has risen steadily in the past 20 years (Figure 1-6). Between 1980 and 2003 the rates of twin births climbed from 18.9 to 31.5 per 1,000 live births. The rates of triplets or higher-order multiple births increased from 37 to 187.4 per 100,000 live births. Multiple births are much more likely than singletons to be born preterm. The rise in multiple births is largely due to the use of ART (in vitro fertilization and other procedures in which in which the egg and the sperm are handled in the laboratory) (NCCDPHP, 2005), which is more frequently used by older women (see Chapter 5 for a discussion). In 2002, approximately 1 percent of infants born were conceived through the use of ART (NCCDPHP, 2005). Although the use of ARTs must be reported to the Centers for Disease Control and Prevention, this is not the case with other fertility treatments not classified as ARTs. These other treatments include those in which only sperm are handled (i.e., intrauterine insemination, which is also known as artificial insemination) or procedures in which a woman takes medication to stimulate egg production without the intention of having the eggs retrieved. The latter procedure is used to improve fertility, but the frequency of use of this technique and the number of births attributable to the use of this technique are not precisely known.

Women in their mid- to late 30s had the highest percentages of births conceived through the use of ART (Figure 1-7). Low birth weight infants are also a result of ART treatments, separate for their increased risk of preterm birth. Interestingly, singletons conceived through the use of ART have an increased risk of preterm birth than naturally conceived singletons. The incidence of multiple births is also related to the trend to delay childbearing. The reasons for this are unknown. Older mothers also are more likely than younger mothers to naturally conceive multiple fetuses.

15 -

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FIGURE 1-6 Multiple births as the number per 1,000 live births, 1980 to 2002. SOURCE: CDC (2003c).

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