Fertile Ground

Kevin Lewis

December 12, 2009

The End of "Lowest-Low" Fertility?

Joshua Goldstein, Tomáš Sobotka & Aiva Jasilioniene
Population and Development Review, December 2009, Pages 663-699

Total fertility rates fell to previously unseen levels in a large number of countries beginning in the early 1990s. The persistence of TFRs below 1.3 raised the possibility of rapid population aging and decline. We discuss the recent widespread turnaround in so-called lowest-low-fertility countries in Europe and East Asia. The number of countries with TFRs below 1.3 fell from 21 in 2003 to five in 2008. Moreover, the upturn in the TFR was not confined to lowest-fertility countries, but affected the whole developed world. We explore the demographic explanations for the recent rise in TFRs stemming from fertility timing effects as well as economic, policy, and social factors. Although the current economic downturn may suppress TFRs in the short run, we conclude that formerly lowest-low-fertility countries will continue to see increases in fertility as the transitory effects of shifts to later childbearing become less important.


The fertility effect of catastrophe: U.S. hurricane births

Richard Evans, Yingyao Hu & Zhong Zhao
Journal of Population Economics, January 2010, Pages 1-36

Anecdotal evidence has suggested increased fertility rates resulting from catastrophic events in an area. In this paper, we measure this fertility effect using storm advisory data and fertility data for the Atlantic and Gulf-coast counties of the USA. We find that low-severity storm advisories are associated with a positive and significant fertility effect and that high-severity advisories have a significant negative fertility effect. As the type of advisory goes from least severe to most severe, the fertility effect of the specific advisory type decreases monotonically from positive to negative. We also find some other interesting demographic effects.


Body Size at Birth and Same-Sex Marriage in Young Adulthood

Morten Frisch & Slobodan Zdravkovic
Archives of Sexual Behavior, forthcoming

An unexplained excess of overweight has been reported among lesbians. In contrast, reports suggest that gay men may be, on average, slightly lighter and shorter than heterosexual men. We studied associations between weight, length, and body mass index (BMI) at birth and same-sex marriage in young adulthood among 818,671 Danes. We used linear regression to calculate differences in mean body measures at birth and Poisson regression analysis to calculate confounder-adjusted incidence rate ratios (IRR) of same-sex marriage according to body measures at birth. Overall, 739 persons entered same-sex marriage at age 18-32 years during 5.6 million person-years of follow-up. Birth year-adjusted mean body measures at birth were similar for same-sex married and other women. However, same-sex marriage rates were 65% higher among women of heavy birth weight (IRR = 1.65; 95% CI = 1.18-2.31, for ≥4000 vs. 3000-3499 g, p = .02), and rates were inversely associated with birth length (p trend = .04). For same-sex married men, birth year-adjusted mean weight (-72 g, p = .03), length (-0.3 cm, p = .04), and BMI (-0.1 kg/m2, p = .09) at birth were lower than for other Danish men. Same-sex marriage rates were increased in men of short birth length (IRR = 1.45; 95% CI = 1.01-2.08, for ≤50 vs. 51-52 cm), although not uniformly so (p trend = .16). Our population-based findings suggest that overweight in lesbians may be partly rooted in constitutional factors. Novel findings of smaller average body measures at birth in same-sex marrying men need replication. Factors affecting intrauterine growth may somehow influence sexual and partner-related choices in adulthood.


Family Change and Continuity in Iran: Birth Control Use Before First Pregnancy

Mohammad Jalal Abbasi-Shavazi, Philip Morgan, Meimanat Hossein-Chavoshi & Peter McDonald
Journal of Marriage and Family, December 2009, Pages 1309-1324

Using data from the 2002 Iran Fertility Transition Survey, we examined birth control use between marriage and first pregnancy. We focused on the post-1990 increase in birth control use and develop two explanations. The first posits that birth control use reflects a new marriage form, the conjugal marriage, which places a heightened value on the spousal relationship while deemphasizing the centrality of parenthood. A second explanation stresses the use of a new resource, effective birth control, within an Iranian-Islamist view of marriage. Key to this explanation is the role of the state-Iranian political/religious actors encourage early marriage and the use of birth control. Although the explanations could be complementary, evidence provides more support for the latter.


Boys live dangerously in the womb

Johan Eriksson, Eero Kajantie, Clive Osmond, Kent Thornburg & David Barker
American Journal of Human Biology, forthcoming

The growth of every human fetus is constrained by the limited capacity of the mother and placenta to deliver nutrients to it. At birth, boys tend to be longer than girls at any placental weight. Boy's placentas may therefore be more efficient than girls, but may have less reserve capacity. In the womb boys grow faster than girls and are therefore at greater risk of becoming undernourished. Fetal undernutrition leads to small size at birth and cardiovascular disorders, including hypertension, in later life. We studied 2003 men and women aged around 62 years who were born in Helsinki, Finland, of whom 644 had hypertension: we examined their body and placental size at birth. In both sexes, hypertension was associated with low birth weight. In men, hypertension was also associated with a long minor diameter of the placental surface. The dangerous growth strategy of boys may be compounded by the costs of compensatory placental enlargement in late gestation. In women, hypertension was associated with a small placental area, which may reduce nutrient delivery to the fetus. In men, hypertension was linked to the mothers' socioeconomic status, an indicator of their diets: in women it was linked to the mothers' heights, an indicator of their protein metabolism. Boys' greater dependence on their mothers' diets may enable them to capitalize on an improving food supply, but it makes them vulnerable to food shortages. The ultimate manifestation of their dangerous strategies may be that men have higher blood pressures and shorter lives than women.


Infant sex predicts breast milk energy content

Camille Powe, Cheryl Knott & Nancy Conklin-Brittain
American Journal of Human Biology, January/February 2010, Pages 50-54

During human evolutionary history, and for many around the world, breast milk is the primary source of nutritional energy for infants. Variation in breast milk quality might logically have important effects on infant health, growth, and development, yet the sources of this variation remain largely unelucidated. We quantified nutrient and energy content of breast milk from 25 healthy, well-nourished Massachusetts mothers with infants aged 2-5 months. We examined several potential sources of variation in milk quality, particularly feeding patterns, infant sex, and maternal breast growth during pregnancy. After controlling for time since last feeding, a known correlate of milk composition, we found that mothers of male infants produced milk that had 25% greater energy content than mothers of female infants (P < 0.001). Change in maternal bra cup size during pregnancy was associated with 16.17 kcal/100 ml greater energy content of milk (P = 0.009), but was not significant after taking infant sex into account. Greater nutritional investment in sons may account for the greater observed growth rates in male compared to female infants.


Why Do Mothers Breastfeed Girls Less Than Boys? Evidence and Implications for Child Health in India

Seema Jayachandran & Ilyana Kuziemko
NBER Working Paper, June 2009

Medical research indicates that breastfeeding suppresses post-natal fertility. We model the implications for breastfeeding decisions and test the model's predictions using survey data from India. First, we find that breastfeeding increases with birth order, since mothers near or beyond their desired total fertility are more likely to make use of the contraceptive properties of nursing. Second, given a preference for having sons, mothers with no or few sons want to conceive again and thus limit their breastfeeding. We indeed find that daughters are weaned sooner than sons, and, moreover, for both sons and daughters, having few or no older brothers results in earlier weaning. Third, these gender effects peak as mothers approach their target family size, when their decision about future childbearing (and therefore breastfeeding) is highly marginal and most sensitive to considerations such as ideal sex composition. Because breastfeeding protects against water- and food-borne disease, our model also makes predictions regarding health outcomes. We find that child-mortality patterns mirror those of breastfeeding with respect to gender and its interactions with birth order and ideal family size. Our results suggest that the gender gap in breastfeeding explains 14 percent of excess female child mortality in India, or about 22,000 "missing girls" each year.


Decline in Sex Ratio at Birth After Bam (Kerman Province, Southern Iran) Earthquake

Mostafa Saadat
Journal of Biosocial Science, November 2008, Pages 935-937

On 26th December 2003, a severe earthquake hit the city of Bam in Kerman province, southern Iran. It destroyed around 90% of houses and at least 60% of the public buildings, and claimed the lives of more than 20,000 persons. To investigate whether acute stress caused by the Bam earthquake could alter the sex ratio at birth (SRB) 6-12 months later, the present study was done. The number of live births by sex was obtained from the National Organization for Civil Registration (Kerman province). The SRB was expressed as the male proportion. A prominent decline in the SRB (̃0·467) 11 months after the earthquake was observed (χ2=6·68, df=1, p=0·009). There was no significant difference between Bam and Kerman province (excluding Bam) for SRB (χ2=0·44, df=1, p=0·51) for a period of 33 month before the earthquake (from April 2001 to December 2003). It might be concluded that psychological tensions and stress are associated with a decrease in SRB.


Impact of BRCA mutations on female fertility and offspring sex ratio

Roxana Moslehi, Ranjana Singh, Lawrence Lessner & Jan Friedman
American Journal of Human Biology, forthcoming

Positive selection for inherited mutations in breast and ovarian cancer predisposing genes, BRCA1 and BRCA2, may contribute to the high frequency of BRCA mutations among the Ashkenazi Jewish population. Impact of BRCA mutations on fertility has not been generally explored in epidemiologic studies. There are reports of distorted sex ratios in BRCA carrier families but these findings have been attributed to bias. We investigated the effect of BRCA mutations on female fertility and offspring sex ratio in a study of 260 Ashkenazi Jewish women with ovarian cancer and 331 controls, unselected for age or family history of the disease. Pregnancy success was similar for 96 mutation carrier (0.84) and 164 noncarrier cases (0.87) and controls (0.83). After adjusting for covariates, there were no significant differences between BRCA carrier and noncarrier cases and controls with regards to fertility, despite lower pregnancy rates among all cases compared to controls (P = 0.0049). Male/female sex ratios were significantly lower among offspring of carriers (0.71) than offspring of noncarriers (0.95) or those of the controls (0.99). Comparisons among the three groups yielded statistically significant distortion against males among the offspring of known and obligate BRCA carriers compared to noncarriers (OR = 0.74, 95% CI:0.55-0.99) and controls (OR = 0.71, 95% CI:0.54-0.94). In conclusion, we did not find evidence for an effect of BRCA mutations on female fertility. We found a significant excess of females among the offspring of female carriers of BRCA1 and BRCA2 mutations. Potential contribution of observed sex ratio distortions to positive selection for BRCA mutations may warrant further investigation.


Development, Modernization, and Childbearing: The Role of Family Sex Composition

Deon Filmer, Jed Friedman & Norbert Schady
World Bank Economic Review, Fall 2009, Pages 371-398

Does the sex composition of existing children in a family affect fertility behavior? An unusually large data set, covering 64 countries and some 5 million births, is used to show that fertility behavior responds to the presence-or absence-of sons in many regions of the developing world. The response to the absence of sons is particularly large in Central Asia and South Asia. Modernization does not appear to reduce this differential response. For example, in South Asia the fertility response to the absence of sons is larger for women with more education and has been increasing over time. The explanation appears to be that a latent demand for sons is more likely to manifest itself when fertility levels are low. As a result of this differential fertility behavior, girls tend to grow up with significantly more siblings than do boys, with potential implications for their well-being when quantity-quality tradeoffs result in fewer material and emotional resources allocated to children in larger families.


Son Preference and the Persistence of Culture: Evidence from Asian Immigrants to Canada

Douglas Almond, Lena Edlund & Kevin Milligan
NBER Working Paper, October 2009

Sex ratios at birth are above the biologically normal level in a number of Asian countries, notably India and China. Standard explanations include poverty and a cultural emphasis on male offspring. We study Asian immigrants to Canada using Census data, focussing on sex ratios across generations and religious groups. We find sex ratios to be normal at first parity, but rising with parity if there were no previous son. Since these immigrants are neither poor nor live in a society tolerant of sex discrimination/sex selection, our findings are more consistent with a preference for sons per se (and not for sons as a means to, e.g., old age support). Additionally, we uncover strong differences by religious affiliation that align with historical differences in doctrine concerning infanticide. Comparing across generations of Asian immigrants, we find fertility responds strongly to the sex composition of older children for first generation families. For the second generation, expression of son preference through the fertility channel is muted whereas sex selection seems to persist.


Poor Prognosis with In Vitro Fertilization in Indian Women Compared to Caucasian Women Despite Similar Embryo Quality

Lora Shahine, Julie Lamb, Ruth Lathi, Amin Milki, Elizabeth Langen & Lynn Westphal
PLoS ONE, October 2009, Pages e7599

Background: Disease prevalence and response to medical therapy may differ among patients of diverse ethnicities. Poor outcomes with in vitro fertilization (IVF) treatment have been previously shown in Indian women compared to Caucasian women, and some evidence suggests that poor embryo quality may be a cause for the discrepancy. In our center, only patients with the highest quality cleavage stage embryos are considered eligible for extending embryo culture to the blastocyst stage. We compared live birth rates (LBR) between Indian and Caucasian women after blastocyst transfer to investigate whether differences in IVF outcomes between these ethnicities would persist in patients who transferred similar quality embryos.

Methodology/Principal Findings: In this retrospective cohort analysis, we compared IVF outcome between 145 Caucasians and 80 Indians who had a blastocyst transfer between January 1, 2005 and June 31, 2007 in our university center. Indians were younger than Caucasians by 2.7 years (34.03 vs. 36.71, P = 0.03), were more likely to have an agonist down regulation protocol (68% vs. 43%, P<0.01), and were more likely to have polycystic ovarian syndrome (PCOS), although not significant, (24% vs. 14%, P = 0.06). Sixty eight percent of Indian patients had the highest quality embryos (4AB blastocyst or better) transferred compared to 71% of the Caucasians (P = 0.2). LBR was significantly lower in the Indians compared to the Caucasians (24% vs. 41%, P<0.01) with an odds ratio of 0.63, (95%CI 0.46-0.86). Controlling for age, stimulation protocol and PCOS showed persistently lower LBR with an adjusted odds ratio of 0.56, (95%CI 0.40-0.79) in the multivariate analysis.

Conclusions/Significance: Despite younger age and similar embryo quality, Indians had a significantly lower LBR than Caucasians. In this preliminary study, poor prognosis after IVF for Indian ethnicity persisted despite limiting analysis to patients with high quality embryos transferred. Further investigation into explanations for ethnic differences in reproduction is needed.


Preconception Motivation and Pregnancy Wantedness: Pathways to Toddler Attachment Security

Warren Miller, Marjorie Sable & Jonathon Beckmeyer
Journal of Marriage and Family, December 2009, Pages 1174-1192

This research was designed to increase our understanding of how the motivational antecedents to childbearing and emotional responses to pregnancy affect the subsequent attachment bond of a toddler to his or her mother. Using a sample of 1,364 mothers and their newborns from the Study of Early Child Care, we tested a mother-child influence framework hypothesized to mediate between the mother's preconception motivation and pregnancy wantedness and her child's security of attachment at 24 months. Almost all elements of the mediational sequence were confirmed in a structural equation model. We observed 2 unmediated pathways to attachment security, 1 from preconception motivation and 1 from parenting stress during infancy, and suggest interpretations of these unexpected pathways.


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