Why twin pregnancies are more successful at advanced than young maternal age? A potential role of 'terminal reproductive investment'

Higher Rates of Twinning Among Repeat Vs First-Time Teenage and Young Adult Mothers in the United States, 2009-2018

STUDY OBJECTIVE

Few studies examining predictors of twinning consider younger mothers who do not use assisted reproductive technologies (ART). Higher parity is associated with greater odds of having a twin birth, but it is unclear whether this association is present among young women. We tested the hypothesis that the rates and odds of twinning would increase with parity among teenage and young adult mothers who did not use ART.

DESIGN, SETTING, AND PARTICIPANTS

We conducted a retrospective, population-based cohort study using 2009-2018 United States National Vital Statistics data on 11,383,370 (58.94% first, 41.06% repeat) births to adolescent and adult women aged 15-24 years.

INTERVENTIONS

None.

MAIN OUTCOME MEASURES

Rates and odds of twinning by parity among teenage (15-19 years) and young adult (20-24 years) mothers.

RESULTS

The adjusted twin birth rate among first-time teenage mothers was 13.28 per 1000 births compared to 16.62 twins per 1,000 births among repeat teenage mothers. This difference by parity was present but smaller among mothers aged 20-24 (18.31 vs 21.44 twins per 1000 births for first-time and repeat young adult mothers, respectively).

CONCLUSION

Repeat young mothers, particularly teenage mothers, are a high-risk group in terms of infant and maternal health outcomes; their higher rate of twinning puts them at an additional risk for adverse birth outcomes. Practitioners may counsel young mothers at risk for subsequent unintended pregnancy on elevated risk of twinning.

Perinatal outcome of twin pregnancies according to maternal age

Objective

To investigate the perinatal outcomes of twin pregnancies according to maternal age.

Methods

This is a retrospective cohort study of twin pregnancies delivered ≥24 weeks' gestation at a tertiary academic hospital from 1995 to 2016. Subjects were categorized into 5 groups according to maternal age: <25, 25–29, 30–34, 35–39, and ≥40 years. Maternal and neonatal outcomes of each maternal age group were analyzed using the Jonckheere-Terpstra test and the linear-by-linear association test.

Results

A total of 1,936 twin pregnant women were included, of which 47 (2.4%), 470 (24.3%), 948 (49.0%), 417 (21.5%), and 54 (2.7%) women were aged <25, 25–29, 30–34, 35–39, and ≥40 years, respectively. Higher maternal age was significantly associated with a higher rate of dichorionic twins and a higher risk of gestational diabetes and placenta previa. However, rates of preterm labor, preterm premature rupture of membranes, cervical incompetence, preterm delivery, preeclampsia, placenta abruption, and cesarean section were not associated with maternal age. Birth weight increased and the rate of admission to the neonatal intensive care unit (NICU) decreased with older maternal age, but other neonatal outcomes did not change with age. Maternal age was significantly associated with a lower rate of NICU admission after controlling for potential confounding factors in multivariable analysis.

Conclusion

Advanced maternal age in twin pregnancies was associated with increased risk of gestational diabetes, placenta previa, and higher birth weight but a lower rate of NICU admission. However, other outcomes were not significantly associated with maternal age.

Human Dizygotic Twinning: Evolutionary-Based Explanations/Twin Research Reviews: Conjoined Twins in a Triplet Pregnancy; Double Embryo Transfer; Health Anxiety; Delayed Delivery of the Second Twin/In the Media: Digital Marketing Twins; Bereaved Twin at March for Our Lives; Exchange of Places; Football Players Reunite as Patriots

Human twinning poses a conundrum for researchers, given that the female uterus is optimally designed to carry a single fetus. Evolutionary explanations of dizygotic (DZ or fraternal) twinning provide insight and understanding of this reproductive event. This review is followed by summaries of recent twin research and reports concerning twins in a triplet pregnancy, double embryo transfer; a twin study of health anxiety, and delayed delivery of the second twin. Twin events reported in the media include a pair of digital marketing twins, a speech by a bereaved twin at the March for Our Lives in Washington, D.C., twins who exchanged places, and a same-team reunion of twin football players.

Access to resources shapes maternal decision making: evidence from a factorial vignette experiment

The central assumption of behavioral ecology is that natural selection has shaped individuals with the capacity to make decisions that balance the fitness costs and benefits of behavior. A number of factors shape the fitness costs and benefits of maternal care, but we lack a clear understanding how they, taken together, play a role in the decision-making process. In animal studies, the use of experimental methods has allowed for the tight control of these factors. Standard experimentation is inappropriate in human behavioral ecology, but vignette experiments may solve the problem. I used a confounded factorial vignette experiment to gather 640 third-party judgments about the maternal care decisions of hypothetical women and their children from 40 female karo Batak respondents in rural Indonesia. This allowed me to test hypotheses derived from parental investment theory about the relative importance of five binary factors in shaping maternal care decisions with regard to two distinct scenarios. As predicted, access to resources--measured as the ability of a woman to provide food for her children--led to increased care. A handful of other factors conformed to prediction, but they were inconsistent across scenarios. The results suggest that mothers may use simple heuristics, rather than a full accounting for costs and benefits, to make decisions about maternal care. Vignettes have become a standard tool for studying decision making, but have made only modest inroads to evolutionarily informed studies of human behavior.

A two-resource model of terminal investment

The most widely known theoretical basis for the hypothesis of terminal investment is the classic model by George C. Williams (1966). Although this model predicts that reproductive effort (i.e. the proportion of available resources devoted to reproduction) increases with decreasing reproductive value, it implies that reproductive allocation in absolute terms should remain stable. This contrasts with the empirical evidence on terminal investment reported to date: the vast majority of positive case studies report an increase in some aspect of reproductive allocation in absolute terms. Also, a substantial number of studies have failed to record terminal investment, despite expectations. Here, I present a simple conceptual model which explains such results. I argue that to explain terminal investment, an organism's reproductive capacity must not be considered as a common pool of resources (often described by the term 'reproductive value'), but as a set of different resources which are not easily convertible to each other, and should be exhausted in balance. Thus, if one resource accidentally decreases, in response, the others must be expended at higher rate. To test this model, each reproductive allocation should be measured in a more specific currency (or currencies) than traditional 'reproductive effort'. The model is consistent with both the positive and the negative case reports on terminal investment.