Is preterm delivery genetically determined?

Inequities in Adverse Maternal and Perinatal Outcomes: The Effect of Maternal Race and Nativity

OBJECTIVES

To evaluate the effect of race and ethnicity on differences in maternal and perinatal outcomes among U.S.-born and foreign-born women, as well as racial and ethnic disparities in outcomes within these groups.

METHODS

This retrospective study analyzed singleton pregnancies (n = 11,518) among women delivering at Boston Medical Center from January 2010-March 2015. Outcomes of interest included preterm birth, early preterm birth, cesarean delivery, hypertensive disorders, diabetes, low birth weight at term (LBW, < 2500 g), NICU admission and intrauterine fetal demise (IUFD). Prevalence ratios and 95% confidence intervals comparing outcomes between U.S.- and foreign-born women were calculated and stratified by race. Obstetric outcomes among Black and Hispanic women were compared to those of white women within both U.S.- and foreign-born groups.

RESULTS

Preterm birth, hypertensive disorders, LBW and NICU admission were more likely to occur among U.S.-born women and their neonates compared to foreign-born women. Controlling for sociodemographic characteristics did not significantly impact these disparities. Among foreign-born women, Black women had a higher prevalence of many maternal and neonatal complications, while Hispanic women had a lower prevalence of some complications compared to white women. Black woman and infants consistently exhibit worse outcomes regardless of their nativity, while Hispanic women foreign-born women experience less disparate outcomes.

CONCLUSIONS FOR PRACTICE

Overall, women born in the United States are at higher risk of several adverse perinatal outcomes compared to foreign-born women. Racial and ethnic disparities in birth outcomes exist in both groups. However, the complex interplay between biopsychosocial influences that mediate these inequities appear to have different effects among U.S- and foreign- born women. A better understanding of these factors can be used to combat disparities and improve outcomes for all women.

A possible role for the PPARG Pro12Ala polymorphism in preterm birth

The links between preterm birth, low birth weight, and adult vascular/metabolic morbidity remain unclear. Genetic susceptibility of babies related to these three conditions might contribute to this long-term association. We tested whether the Pro12Ala polymorphism of the peroxisome proliferator-activated receptor gamma (PPARG) gene could play a role in birth weight and duration of gestation. We genotyped two independent cross-sectional studies from Northern Ireland (n = 382 and 620). In combined populations, the PPARG Ala12 allele was associated (P = 0.03) with lower birth weight, primarily caused by shorter gestational duration (P = 0.04). The frequency of Ala12 allele carriers was higher (P = 0.027) in the group of individuals born before term (35%, n = 60) than in the group of individuals born at term (22%, n = 942). The odds ratios (95% CI) of preterm birth for Ala12 allele carriers were 1.9 (1.1-3.4), P = 0.022, and 4.2 (1.9-9.7), P = 0.0006 (adjusted for sex, maternal age, and study), when considering 37 or 35 weeks of pregnancy as a threshold for preterm birth, respectively. Interestingly, the same allele was also associated with a moderate decreased risk of miscarriages in mothers. In conclusion, the PPARG Pro12Ala polymorphism might represent a genetic susceptibility factor for preterm birth and constitute a link between preterm birth and metabolic diseases later in life.

The Heritability of Preterm Delivery

OBJECTIVE

To study the heritability of preterm delivery.

METHODS

Women who delivered a singleton infant at less than 36 weeks of gestation were asked about their family history. Twenty-eight families were identified in which the proband had at least five first- or second-degree relatives with preterm delivery. An extensive genealogy database (GenDB) was constructed using more than 9,000 genealogy sources in the public domain (records before 1929). GenDB documents the relationships between more than 17.5 million ancestors and 3.5 million descendants of approximately 10,000 individuals who moved to Utah in the mid 1800s. This database was searched for the names, birth dates, and birthplaces of the four grandparents for each of the 28 probands. Pairwise coefficients of kinship were determined for the 93 preterm delivery grandparents identified, and for sets of 100 individuals born in the 1920s who were randomly selected from the population database.

RESULTS

Probands had a mean of 3.3 grandparents included in this database. The average coefficient of kinship for controls was 1.5 x 10(6) (standard deviation = 0.6 x 10(6)). This measure agrees with previous calculations for the Utah population. The coefficient of kinship for familial preterm delivery grandparents was more than 50 standard deviations higher (3.4 x 10(5) [P < .001]).

CONCLUSION

This study confirms the familial nature of preterm delivery. On average, gravidae randomly selected from our population are 23rd degree relatives, while these preterm delivery probands are eighth-degree relatives. A genome-wide scan using these affected families is underway.

The genetic contribution towards preterm delivery

Indirect evidence supports a possible genetic predisposition towards preterm birth. The recurrence of spontaneous preterm delivery in individual women, families and ethnic groups suggests a long-acting aetiology, consistent with a genetic factor. Genetic contributions from both mother and fetus probably play a role in determining gestational length. Preliminary genetic association studies implicate gene variants of tumor necrosis factor-alpha (TNF-alpha) in preterm birth. Further understanding of a genetic predisposition begins with investigation of the pathogenesis of preterm delivery. Technological advances in the study of the human genome (genomics) and protein complement (proteomics) will allow identification of novel genes and proteins involved in preterm delivery. Insight into the complex gene regulation and protein production in preterm delivery may contribute to an understanding of a genetic basis. A discovered genetic factor may lead to medical breakthroughs and reductions in prematurity, neonatal morbidity and mortality.

Socio-economic disparities in preterm birth: causal pathways and mechanisms

Preterm birth is the leading cause of infant mortality in industrialised societies. Its incidence is greatly increased among the socially disadvantaged, but the reasons for this excess are unclear and have been relatively unexplored. We hypothesise two distinct sets of causal pathways and mechanisms that may explain social disparities in preterm birth. The first set involves chronic and acute psychosocial stressors, psychological distress caused by those stressors, increased secretion of placental corticotropin releasing hormone (CRH), changes in sexual behaviours or enhanced susceptibility to bacterial vaginosis and chorioamnionitis, cigarette smoking or cocaine use, and decidual vasculopathy. The second hypothesised pathway is a gene-environment interaction based on a highly prevalent mutation in the gene for methylenetetrahydrofolate reductase (MTHFR), combined with low folate intake from the diet and from prenatal vitamin supplements, consequent hyperhomocysteinemia, and decidual vasculopathy. We propose to test these hypothesised pathways and mechanisms in a nested case-control study within a prospectively recruited and followed cohort of pregnant women with singleton pregnancies who deliver at one of four Montreal hospitals that serve an ethnically and socio-economically diverse population. Following recruitment during the late first or early second trimester, participating women are seen at 24-26 weeks, when a research nurse obtains a detailed medical and obstetric history; administers several scales to assess chronic and acute stressors and psychological function; obtains blood samples for CRH, red blood cell and plasma folate, homocysteine, and DNA for the MTHFR mutation; and performs a digital and speculum examination to measure cervical length and vaginal pH and to obtain swabs for bacterial vaginosis and fetal fibronectin. After delivery, each case (delivery at < 37 completed weeks following spontaneous onset of labour or prelabour rupture of membranes) and two controls are selected for placental pathological examination, hair analysis of cotinine, cocaine, and benzoylecgonine, and analysis of stored blood and vaginal specimens. Statistical analysis will be based on multiple logistic regression and structural equation modelling, with sequential construction of models of potential aetiological determinants and covariates to test the hypothesised causal pathways and mechanisms. The research we propose should improve understanding of the factors and processes that mediate social disparities in preterm birth. This improved understanding should help not only in developing strategies to reduce the disparities but also in suggesting preventive interventions applicable across the entire socio-economic spectrum.

Genetic factors in preterm delivery

Tens of thousands of children deliver before they are full term each year. Although many social, environmental, and medical risk factors have been suggested, the etiology of a large percentage of preterm labor cases is still unknown. It has been noted for many years that preterm delivery is a condition that runs in families. Evidence concerning its aggregation among families, the recurrent nature of preterm labor, and its differing prevalence between races has led to the suggestion of a genetic cause for preterm delivery. There have been few formal studies to investigate this hypothesis. We suggest that modern molecular biology approaches can reveal the part that genes play in preterm delivery.

Differing birth weight among infants of U.S.-born blacks, African-born blacks, and U.S.-born whites

BACKGROUND

In the United States, the birth weights of infants of black women are lower than those of infants of white women. The extent to which the lower birth weights among blacks are related to social or genetic factors is unclear.

METHODS

We used vital records for 1980 through 1995 from Illinois to determine the distribution of birth weights among infants born to three groups of women -- U.S.-born blacks, African-born blacks, and U.S.-born whites.

RESULTS

The mean birth weight of 44,046 infants of U.S.-born white women was 3446 g, that of 3135 infants of African-born black women was 3333 g, and that of 43,322 infants of U.S.-born black women was 3089 g. The incidence of low birth weight (weight less than 2500 g) was 13.2 percent among infants of U.S.-born black women and 7.1 percent among infants of African-born black women, as compared with 4.3 percent among infants of U.S.-born white women (relative risks, 3.1 and 1.6, respectively). Among the women at lowest risk (those 20 to 39 years old, with 12 years of education for themselves and their spouses, early prenatal care, gravida 2 or 3, and no previous fetal loss), the rate of low birth weight in infants of African-born black women (3.6 percent) was closer to the rate in infants of U.S.-born white women (2.4 percent), and the rate in infants of U.S.-born black women remained high (7.5 percent).

CONCLUSIONS

The birth-weight patterns of infants of African-born black women and U.S.-born white women are more closely related to one another than to the birth weights of infants of U.S.-born black women.

A promoter mutation that increases transcription of the tumor necrosis factor-alpha gene is not associated with preterm delivery

OBJECTIVE

Increased amniotic fluid concentrations of tumor necrosis factor-alpha are observed in women with preterm labor and subsequent preterm birth. We tested whether a mutation in the promoter region of tumor necrosis factor-alpha gene, TNF T2, which increases transcription of the gene, is more frequent in a preterm delivery cohort.

STUDY DESIGN

Deoxyribonucleic acid was extracted from whole blood of 203 women and 44 fetuses delivered at < 37 weeks of estimated gestational age. The polymerase chain reaction was used to amplify the promoter region of the tumor necrosis factor-alpha gene. The resulting polymerase chain product was subjected to allele-specific enzymatic digestion with Nco I. Fragments were size fractionated on a 3% Metaphor agarose gel stained with ethidium bromide. Results were analyzed with use of a chi 2 contingency table.

RESULTS

No statistically significant differences for either the TNF T1 or TNF T2 allele frequencies were found between women or fetuses delivered preterm compared with a control group or previously published allele frequencies.

CONCLUSIONS

The frequency of this tumor necrosis factor-alpha promoter mutation, TNF T2, is not increased in either women or fetuses delivered at < 37 weeks' gestation. Basal levels of tumor necrosis factor-alpha are unlikely to affect a woman's risk of preterm delivery. Tumor necrosis factor-alpha variants should not be used as a predictive test for preterm delivery.

Race and birthweight in biracial infants

OBJECTIVES

The purpose of the study was to determine the role of infant race as a determinant of the Black-White disparity in low birthweight (< 2500 g).

METHODS

Univariate analysis and multivariate logistic regression were performed on Illinois vital records from 1982 and 1983 and on 1980 United States census income data.

RESULTS

Fourteen percent of the infants born to Black mothers and White fathers were of low birthweight, compared with 9% of infants born to White mothers and Black fathers and 6% of a random sample of White infants. Both groups of biracial infants were more likely to have been born to unmarried mothers and to reside in very low-income (< $10,000 per year) census tracts than were White infants. When all confounding variables were entered into a logistic model, the adjusted odds ratio of low birthweight for biracial infants born to Black mothers and White fathers equaled 1.4. When biracial infants born to White mothers and Black fathers were compared with White infants, the adjusted odds ratio of low birthweight equaled 1.0.

CONCLUSIONS

Paternal and consequent infant race does not affect the birthweight distribution of those born to White mothers and Black fathers. Unidentified factors closely related to maternal race underlie the Black-White disparity in infant birthweight.

Are physical activity and employment related to preterm birth and low birth weight?

OBJECTIVE

The purpose of this study was to review the validity of published studies that consider modifying physical activity during pregnancy to decrease preterm births and low birth weight.

STUDY DESIGN

Presented is a critical review of extant literature, both case-control studies and studies of pregnant women followed up in cohort fashion.

RESULTS

Studies both showing and not showing an association between employment and adverse outcome exist. However, no randomized trials have been reported. The most plausible associations exist for prolonged standing, long working hours, and lifting heavy objects.

CONCLUSION

Until the biologic basis of parturition is known, definitive conclusions are probably not possible. Proposals to require pregnancy leave for all employees seem premature. However, physicians must be sensitive to dilemmas faced by some employees and encourage voluntary leave for those placed in positions characterized by occupational fatigue.

Second generation follow-up of the Danish perinatal study women: study design and factors affecting response

To study the mother-infant correlation of fetal growth and duration of pregnancy, women who were born as subjects in the Danish Perinatal Study (1959-61) were traced and interviewed, and the pregnancy and birth records of their children were abstracted. The study population consisted of 159 women who were small-for-gestational age (SGA) at birth, 162 who were preterm, 38 who were both preterm and SGA, and 939 term, appropriately-grown control women. Methods for sample selection, measuring gestational age and fetal growth in both generations, locating and interviewing the women, abstracting the records of their children, and obtaining paternal birth and adult stature are described. A total of 84.5% of the selected women were successfully interviewed; the fraction interviewed did not differ by maternal birth status. The medical records of over 98% of pregnancies to the study women were abstracted, making it possible to study various factors associated with completion of an interview. By a variety of measures, women of higher socio-economic status were more likely to be interviewed. Birthweight and adult weights were available for 63 and 73% of the children's fathers.