Race and mother/fetus mismatch

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

Written by Peter Frost.

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 helped create the environment for your first nine months. The genetic influence is substantial: birthweight, fetal growth and gestational length are 25–40% heritable (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 their pace and timing of fetal development.

A fetus with European parents will come to term a week later, on average, than one with Sub-Saharan African parents (Rushton & Jensen, 2005, p. 264). Since pre-term delivery carries greater risks than post-term delivery (Campbell et al., 1997; Fayed et al., 2022), outcomes should 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.

Empirical evidence

Biracial births, Chicago, 1982–1983. The first relevant 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. The risks were now even greater for 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 eliminated most of the race differences, particularly for very low birthweight infants, but a difference still remained 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.”

For the authors, the difference between Black and White maternal inheritance could not have a genetic cause: “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., 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 for White mother/Black father births improved relative to those for single-race White births, but outcomes for Black mother/White father births did not improve relative to those 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 reached the following conclusion:

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 [low birthweight] and [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 preterm 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 a genetic or environmental cause 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 final 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 births have worse outcomes if the mother is Black than if she is White. Specifically, biracial infants with Black mothers have lower birthweight, higher risk of premature birth, poorer fetal growth, and higher risk of fetal and infant mortality. These differences remain after one adjusts for socioeconomic risk factors, even though the same adjustment equalizes birth outcomes between Black parents and White parents.

The literature leans toward an environmental cause, apparently because many authors assume that a genetic cause should be mediated as much by the father as by the mother. Yet this is clearly untrue. Wombs are specific to women, and the genes that help create wombs are expressed much more in women than in men.

Moreover, if the cause is some unknown factor in the environment of Black mothers, why did it remain unchanged between 1978 and 1997? That period saw considerable social and economic gains for Black women, particularly those in interracial relationships.

A review article has pointed to a possible genetic cause: 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 greater chance of survival in that social environment 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)

A biracial fetus presumably matures more slowly than a Black one, since one of the parents provides genes for a slower pace of fetal development. A conflict may thus arise between a biracial fetus and its womb if the mother is Black. To be specific, the womb may initiate the birth process slightly before the fetus is ready to survive on its own. There is consequently a higher risk of stillbirth and infant mortality.

If we look at outcomes later in life, we see that both pre-term and post-term births are associated with higher risks of behavioral, emotional and developmental problems, especially ADHD. However, these risks are generally higher for those born pre-term than for those born post-term (Marroun et al., 2012).

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, overall, less risky for a newborn baby than a pre-term birth, post-term boys may be at higher risk for adolescent obesity (Beltrand et al., 2012; Schierding et al., 2014). It would be interesting to see whether this risk is higher 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 control appetite and metabolism. A California/Hawaii study found higher levels of obesity in ethnically mixed individuals than in either of the parent populations, 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 decreases progressively for parents who are less and less related to each other. As genetic distance increases, so does the risk of maternal/paternal mismatches in a developing embryo (Frost, 2024).

Peter Frost has a PhD in anthropology from Université Laval. His main research interest is the role of sexual selection in shaping highly visible human traits. Find his newsletter here.

---

Support Aporia with a paid subscription:

You can also follow us on Twitter.

---

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

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

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

Clausson, B., Lichtenstein, P., & Cnattingius, S. (2005). Genetic influence on birthweight and gestational length determined by studies in offspring of twins. BJOG, 107, 375-381. https://doi.org/10.1111/j.1471-0528.2000.tb13234.x

Collins, Jr, J.W., & David, R.J. (1993). Race and birthweight in biracial infants. American Journal of Public Health, 83(8), 1125-1129. https://doi.org/10.2105/AJPH.83.8.1125

Frost, P. (2024). Outbreeding depression: Avenues for further research. Aporia Magazine, February 28.

Gold, K.J., DeMonner, S.M., Lantz, P.M., & Hayward, R.A. (2010). Prematurity and low birth weight as potential mediators of higher stillbirth risk in mixed black/white race couples. Journal of Women's Health, 19(4), 767-773. https://doi.org/10.1089/jwh.2009.1561

Hessol, N. A., Fuentes-Afflick, E., & Bacchetti, P. (1998). Risk of low birth weight infants among black and white parents. Obstetrics & Gynecology, 92(5), 814-822. https://doi.org/10.1016/s0029-7844(98)00310-x

Marroun El, H., Zeegers, M., Steegers, E., van der Ende, J., Schenk, J. J., Hofman, B., Jaddoe, V., Verhulst, F., & Tiemeier, H. (2012). Post-term birth and the risk of behavioural and emotional problems in early childhood. International Journal of Epidemiology, 41(3), 773-781. https://doi.org/10.1093/ije/dys043

Parker, J.D. (2000). Birth Weight Trends Among Interracial Black and White Infants. Epidemiology, 11(3), 242-248. https://doi.org/10.1097/00001648-200005000-00003

Polednak, A.P., & King, G. (1998). Birth weight of US biracial (black-white) infants: regional differences. Ethnicity & Disease, 8(3), 340-349.

Rushton, J.P., & Jensen, A.R. (2005). Thirty years of research on race differences in cognitive ability. Psychology, Public Policy, and Law, 11(2), 235-294. https://psycnet.apa.org/doi/10.1037/1076-8971.11.2.235

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

Sheeder, J., Lezottte, D., & Stevens-Simon, C. (2006). Maternal age and the size of white, black, Hispanic, and mixed infants. Journal of Pediatric and Adolescent Gynecology, 19(6), 385-389. https://doi.org/10.1016/j.jpag.2006.09.012

Simhan, H.N., & Krohn, M.A. (2008). Paternal race and preterm birth. American Journal of Obstetrics and Gynecology, 198(6), 644-e1. https://doi.org/10.1016/j.ajog.2007.11.046

Tan, H., Wen, S.W., Walker, M., & Demissie, K. (2004a). Parental race, birth weight, gestational age, and fetal growth among twin infants in the United States. Early Human Development, 80(2), 153-160. https://doi.org/10.1016/j.earlhumdev.2004.06.005

Tan, H., Wen, S.W., Walker, M., & Demissie, K. (2004b). The effect of parental race on fetal and infant mortality in twin gestations. Journal of the National Medical Association, 96(10), 1337.

Van Den Oord, E.J., & Rowe, D.C. (2001). A step in another direction: Looking for maternal genetic and environmental effects on racial differences in birth weight. Demography, 38(4), 573-576. https://doi.org/10.1353/dem.2001.0040

Yang, J., Carmichael, S.L., Kaidarova, Z., & Shaw, G.M. (2004). Risks of selected congenital malformations among offspring of mixed race-ethnicity. Birth Defects Research Part A: Clinical and Molecular Teratology, 70(10), 820-824. https://doi.org/10.1002/bdra.20054