A mother/fetus mismatch?

When the womb and the fetus have different plans ...

Free woman of color with daughter, New Orleans. Late 18th century (Wikicommons)

Why do births to Black mothers and White fathers have worse outcomes than births to White mothers and Black fathers?

You received the same amount of DNA from your father and your mother, but only your mother provided the womb you grew in. To some extent, your mother’s genes matter more than your father’s, since hers alone helped create the environment for your first nine months. The genetic influence is substantial: heritability is 25-40% for birthweight, fetal growth, and gestational length (Clausson et al., 2005).

Genetic mismatches may therefore arise between the fetus (whose development is determined by both the mother and the father) and the fetal environment (which is created by the mother), especially if the parental populations differ in the pace and timing of fetal development.

If a fetus has European parents, it will come to term a week later, on average, than one with Sub-Saharan African parents (Rushton & Jensen, 2005, p. 264). Sub-Saharan African descent is a risk factor for spontaneous pre-term birth in both the United States and Africa, although “risk” here is defined in terms of European norms of pregnancy duration (Purisch & Gyamfi-Bannerman, 2017).

For a fetus, pre-term delivery is riskier than post-term delivery (Campbell et al., 1997; Fayed et al., 2022). Outcomes should therefore be worse for births to Black mothers and White fathers than for births to White mothers and Black fathers. Indeed, this has been a consistent finding of American studies over the past four decades.

Biracial births, Chicago, 1982-1983

The first study is of all singleton infants born to biracial couples in Chicago from 1982 to 1983 (n = 410 Black mothers, 739 White mothers).

Birthweights were low for 14% of births to Black mothers and White fathers, versus 9% of births to White mothers and Black fathers and 6% of births to White parents.

The authors adjusted the data to control for several risk factors: maternal age, education, marital status, parity (number of pregnancies that reached a viable gestational age), trimester of prenatal care initiation, and median family income of mother's census tract. This adjustment eliminated the difference in risk between White mother/Black father births and White mother/White father births, while maintaining the higher risk of Black mother/White father births:

We found that infants born to Black mothers and White fathers had a 40% higher chance of low-birthweight than did White infants, independent of other risk factors. In contrast, infants born to White mothers and Black fathers had odds of low birthweight equal to those of infants in the general White population when measured risk factors were controlled. (Collins & David, 1993)

The authors concluded that “traditional sociodemographic risk factors do not explain the birthweight disadvantage of biracial infants born to Black mothers. Indeed, our findings suggest that these infants may also be at increased risk for prematurity and intrauterine growth retardation. Other unmeasured variables appear to be exerting a negative effect only in biracial births in which the Black parent is the mother” (Collins & David, 1993)

Biracial births, California, 1992

The next study is of all births to Black, White and biracial couples in California in 1992 (n = 203,815 couples).

The data were adjusted to control for a wider range of risk factors: maternal characteristics (race, age, education, marital status, parity, obstetric history, tobacco use, medical complications, medical insurance and use of prenatal care); paternal characteristics (race, age and education); infant characteristics (gestational age and gender); and community characteristics (median household income from the 1990 US Census).

These adjustments reduced the overall race differences, particularly for very low birthweight infants, but there was now a relatively larger difference between Black mother/White father births and White mother/Black father births: “In the adjusted models analyzing moderately low birth weight infants, only black maternal race was a statistically significant risk factor.” Indeed, having a Black mother seems to explain the low weights of Black mother/White father births just as well as the low weights of Black mother/Black father births.

Yet, for the authors, the cause could not be genetic: “These results imply that black race per se is not a causal risk factor for low birth weight infants but rather that race is a surrogate marker for other causal risk factors, both measured and unmeasured” (Hessol et al., 1998, pp. 819-820).

Biracial births, US, 1978–1997

The next study is of all singleton births to American couples between 1978 and 1997 (n = 50 M).

Between 1978 and 1997, outcomes improved for White mother/Black father births relative to single-race White births, but not for Black mother/White father births relative to single-race Black births (Parker, 2000). The latter finding was unexpected, given the generally better circumstances of biracial infants with White fathers:

By 1994–1997, based on these indices, interracial births to black mothers had lower-risk demographic profiles than interracial births to white mothers. Black mothers with interracial births had more education, were older, were more likely to be foreign born, and were much more likely to be married than white mothers with interracial births. (Parker, 2000)

The author concluded:

This difference between black and white mothers suggests that factors specifically affecting interracial birth differ by maternal race and have changed little for black mothers. Indeed, although demographically a lower-risk group, black mother/white father infants had consistently higher risks of LBW [low birthweight] and VLBW [very low birthweight] than white mother/black father infants. This finding may be due to persistent effects of racial discrimination throughout childhood and adulthood or factors unique to black women, regardless of the race of their infant’s father. (Parker, 2000)

White, Black, Hispanic, and mixed-race births, Colorado, 1989–2000

The next study is of all singleton births to couples in Colorado between 1989 and 2000 (n = 91,061). Mothers were healthy, non-smoking, non-substance-using women who were pregnant for the first time.

Infants with Black mothers were smaller for gestational age (SGA) than infants with White mothers, while father’s race had no effect:

It is also noteworthy that paternal race/ethnicity entered the model for infant birth weight but not the model for SGA delivery. This suggests that paternal race/ethnicity contributes to normal variation in fetal growth but not to the clinically significant aberrations in fetal growth that are responsible for racial/ethnic disparities in neonatal morbidity and mortality in the US. (Sheeder et al., 2006, p. 387)

As in the previous study, the authors ruled out a genetic cause: “The negative findings, with respect to the effect of paternal race/ ethnicity on the risk of SGA delivery, also increase confidence that the effects of maternal race/ethnicity reflect acquired, not genetic, traits” (Sheeder et al., 2006, p. 387)

Twin biracial births, US, 1995–1997

The next study is of all twins born to biracial parents in the US from 1995 to 1997 (n = 702 Black mothers, 1,923 White mothers).

Black mother/White father infants were at higher risk than White mother/Black father infants for very low birthweight, very pre-term birth, fetal growth restriction, and both fetal and infant mortality.

Unfortunately, the authors did not distinguish between identical and fraternal twins, nor did they adjust for socioeconomic risk factors. This study therefore cannot tell us whether genetics or environment better explains the differences between Black and White maternal inheritance (Tan et al., 2004a; Tan et al., 2004b).

Biracial births, US, 1991

The next study is of all births to biracial American couples in 1991 (n = 50,980).

Low birthweight was 31% more frequent among Black mother/White father births (8.4%) than among White mother/Black father births (6.4%). The difference was smaller in the Northeast of the country, perhaps because a higher proportion of the White mothers were Puerto Ricans with some Black admixture (Polednak & King, 1998).

Biracial fetal deaths, California, 1998–2002

The next study is of all stillbirths to biracial couples in California between 1998 and 2002 (n = 0.01 million Black mothers, 0.03 million White mothers).

Risk of death was higher for biracial fetuses with Black mothers/White fathers than for those with White mothers/Black fathers (Gold et al., 2010).

Biracial births, US, 2001

The last study is of all births to biracial American couples in 2001 (n = 10,955 Black mothers, 42,609 White mothers).

“In black-white couples, the odds of preterm birth were greater if the mother was black … than if the father was black” (Simhan & Krohn, 2008).

Discussion

In sum, biracial infants with Black mothers have worse birth outcomes than those with White mothers, i.e., lower birthweight, higher rate of premature birth, poorer fetal growth, and higher fetal and infant mortality. These differences remain after adjustment for socioeconomic risk factors, even though the same adjustment equalizes birth outcomes between infants born to White mothers and Black fathers and those born to White mothers and White fathers.

If the cause is an environmental factor that adversely affects Black mothers, why did it remain unchanged between 1978 and 1997? That period saw major social and economic gains for Black women, particularly those in interracial relationships.

A review article has pointed to a possible genetic factor: the maternal C825T allele of the GNB3 gene (Van Den Oord & Rowe, 2001). This allele lowers birth weight and is much less frequent in Europeans (about 30%) than in Africans and African Americans (up to 80%). Because infants receive less paternal care in the highly polygynous societies of Sub-Saharan Africa, a developing fetus may have a better chance of survival if it matures faster:

On average, Black babies are born a week earlier than White babies, yet they are more mature as measured by pulmonary function, amniotic fluid, and bone development. In the United States, 51% of Black children have been born by week 39 of pregnancy compared with 33% of White children. Black African babies, even those born to mothers in the professional classes, are also born earlier than White babies. (Rushton & Jensen, 2005, p. 264)

If the cause is a genetic factor, a biracial fetus should mature more slowly than a Black one, since one of the parents has genes for slower fetal development. A conflict may thus arise between a biracial fetus and its womb if the mother is Black. Specifically, the womb may initiate the birth process before the fetus can survive on its own, thus increasing the risks of stillbirth and infant mortality.

This is suggested by the first of the above studies on biracial infants. Those born to Black mothers and White fathers, as opposed to White mothers and Black fathers, were not simply born earlier. They were also at increased risk for prematurity, uterine growth retardation, and other adverse effects (Collins & David, 1993).

A biracial fetus in a White womb has fewer risks. It is already mature enough to survive on its own when the womb initiates the birth process. Consequently, post-term births incur fewer risks than pre-term births (Marroun et al., 2012), with the possible exception of a higher risk of obesity for adolescent post-term boys (Beltrand et al., 2012; Schierding et al., 2014).

What about infants born to African immigrants?

Some authors still lean toward an environmental cause for the lower birthweight and higher pre-term birth rate of African Americans. This view is supported by two studies that found a higher birthweight and lower pre-term birth rate for infants born to African immigrants than for those born to African Americans. Since both groups presumably come from the same gene pool, any differences between the two must be environmental in nature.

The first study showed that infants of African immigrants weighed more than those of African Americans, although both groups still weighed less than infants of Euro Americans (David & Collins, 1997). Clearly, low African American birthweights have a partly environmental cause. This was shown by Hessol et al. (1998) when they adjusted their data for socioeconomic factors and thereby reduced the difference in risk for moderately low birthweight between African American births and Euro American births. Nonetheless, the first group still had a 60% higher risk than the second. This residual probably corresponds to the maternal genetic factor that likewise explains the earlier timing and lower weight of Black mother/White father births.

The second study showed that the pre-term birth rate did not differ at all between infants of Euro Americans and those of African immigrants, after adjustment for socioeconomic differences (Braveman et al., 2024). This study, however, had participants from the entire African continent, including Algeria, Djibouti, Egypt, Eritrea, Ethiopia, and Libya. No information is given on the number of participants from each African country, but we do have information on the African immigrant community in California, where the study was conducted. The community’s leading country of origin is Ethiopia, followed by Nigeria, Egypt, and South Africa (Reimers, 2005, p. 243). Thus, most of the participants were probably born outside West Africa, the main ancestral region of African Americans. So we are not looking at the same gene pool.

In addition, most African immigrants to the U.S. have some college education, 69% vs. 63% for Americans in general, partly because many come as students and partly because immigrant selection favors the highly educated (Anderson & Connor, 2018). African immigrants may thus be selected for slower life history and, hence, slower fetal development. This selection would not fully disappear after adjustment for socioeconomic factors, since this adjustment is based on data from American society. In African societies, the college-educated differ much more from the general population because proportionately fewer people go on to higher education.

The same point applies to the other studies that adjusted for socioeconomic factors. Such adjustment gives more weight to those African Americans who have slower life history and, hence, a different genetic makeup. Race differences are therefore reduced, since Euro Americans are now being compared with African Americans who have a more similar life history.

Avenues for future research

Conflicts between maternal and fetal programming may affect fetal development in other ways. According to a study of malformed infants born to biracial couples in California between 1989 and 2000 (n = 50 Black mothers, 150 White mothers), biracial infants are at higher risk for cleft palates, hypospadias, and polydactyly when the mother is Black than when she is White. On the other hand, the risk of hypertrophic pyloric stenosis is lower when she is Black (Yang et al., 2004). This is admittedly a single study with a limited number of cases.

Although a post-term birth is generally less risky for a newborn baby than a pre-term birth, adolescent post-term boys may be at higher risk for obesity (Beltrand et al., 2012; Schierding et al., 2014). It would be interesting to see whether this outcome is worse for biracial boys with White mothers than for those with Black mothers.

Finally, fetal development may be disrupted not only by maternal/fetal conflicts but also by maternal/paternal conflicts, perhaps in genes that influence appetite and metabolism. A California/Hawaii study found higher levels of obesity in ethnically mixed individuals than in either parental population, even after controlling for psychosocial and lifestyle factors, i.e., age, marital status, education, and smoking status:

For instance, the prevalence of overweight/obesity in five ethnic admixtures—Asian/white, Hawaiian/white, Hawaiian/Asian, Latina/white, and Hawaiian/Asian/white ethnic admixtures—was significantly higher (P < 0.0001) than the average of the prevalence estimates for their component ethnic groups.

… Controlling for psychosocial and lifestyle factors did not attenuate the differences in prevalence between ethnic admixtures and monorace adults. However, a high caloric intake (e.g., calories from fat and alcohol) and exercise did modestly decrease this difference, and could be important factors for future interventions to control obesity in mixed-race individuals. (Albright et al., 2008)

This sort of mismatch may explain why fertility seems to peak in marriages between third or fourth cousins and then falls progressively for parents who are decreasingly less related to each other. As genetic distance increases, so does the risk of maternal/paternal mismatches in a developing embryo (Frost, 2024).

References

Albright, C. L., Steffen, A., Wilkens, L. R., Henderson, B. E., & Kolonel, L. N. (2008). The prevalence of obesity in ethnic admixture adults. Obesity, 16 (5), 1138–1143. https://doi.org/10.1038/oby.2008.31

Anderson, M., & Connor, P. (2018). Sub-Saharan African Immigrants in the U.S. Are Often More Educated Than Those in Top European Destinations, Pew Research Center, April 24. https://www.pewresearch.org/internet/2018/04/24/sub-saharan-african-immigrants-in-the-u-s-are-often-more-educated-than-those-in-top-european-destinations/

Beltrand, J., Soboleva, T. K., Shorten, P. R., Derraik, J. G., Hofman, P., Albertsson-Wikland, K., ... & Cutfield, W. S. (2012). Post-term birth is associated with greater risk of obesity in adolescent males. The Journal of Pediatrics, 160(5), 769-773. https://doi.org/10.1016/j.jpeds.2011.10.030

Braveman, P., Heck, K., Dominguez, T. P., Marchi, K., Burke, W., & Holm, N. (2024). African immigrants’ favorable preterm birth rates challenge genetic etiology of the Black-White disparity in preterm birth. Frontiers in Public Health, 11, 1321331. https://doi.org/10.3389/fpubh.2023.1321331

Campbell, M. K., Østbye, T., & Irgens, L. M. (1997). Post-term birth: risk factors and outcomes in a 10-year cohort of Norwegian births. Obstetrics & Gynecology, 89(4), 543-548. https://doi.org/10.1016/S0029-7844(97)00049-5

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David, R. J., & Collins Jr, J. W. (1997). Differing birth weight among infants of US-born blacks, African-born blacks, and US-born whites. New England Journal of Medicine, 337(17), 1209-1214. https://doi.org/10.1056/nejm199710233371706

Fayed, A., Wahabi, H. A., Esmaeil, S., Elmorshedy, H., & AlAniezy, H. (2022). Preterm, early term, and post-term infants from Riyadh mother and baby multicenter cohort study: The cohort profile. Frontiers in Public Health, 10, 928037. https://doi.org/10.3389/fpubh.2022.928037

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Schierding, W., O’Sullivan, J. M., Derraik, J. B., & Cutfield, W. S. (2014). Genes and post-term birth: late for delivery. BMC Research Notes, 7, 1-5. https://doi.org/10.1186/1756-0500-7-720

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