The trend in the relationship of advanced maternal age to preterm birth and low birthweight

Abstract

Purpose: As women in developed countries tend to delay childbearing, it becomes more important to understand the relationship of advanced maternal age to birth outcomes. We aimed to estimate the trend in the relationship of advanced maternal age to preterm birth and low birthweight.

Materials and methods: We analysed 4,264,417 ethnically homogeneous, singleton firstborns, born in hospitals to married couples in South Korea in 1997–2014. We regressed an indicator for preterm birth or low birthweight on advanced maternal age, the baby’s sex, advanced paternal age, and a set of socioeconomic status (SES) variables by year. We then collected the coefficient on advanced maternal age and charted its trend. We repeated the same procedure for 4,153,313 second- and third births.

Results: When we controlled for only the baby’s sex, the relationship between advanced maternal age and preterm birth dramatically weakened in the 2000s and slightly more thereafter: being an older mother was related to a 3.5% point increase in preterm birth in the late 1990s, but this figure decreased to less than 2% points by the early 2010s. Controlling for advanced paternal age slightly decreased the relationship and controlling for SES hardly affected the relationship. We obtained almost the same results for low birthweight. Second- and third-borns exhibited a declining, much weaker influence of advanced maternal age on the birth outcomes.

Conclusions: In relative terms, mothers of advanced age were more likely to deliver preterm and low birthweight babies than younger mothers. In absolute terms, however, the risk was small in the 1990s and much smaller in the early 2010s.

Chinese abstract

目的：随着发达国家女性推迟生育, 了解高龄产妇与妊娠结局的关系越来越重要。我们研究目的是评估高龄产妇与早产及低出生体重儿关系的趋势。

材料与方法：我们分析了1997-2014年间4,264,417名同种人、单胎初产、在医院生产的韩国已婚夫妇的婴儿。我们回顾一个对早产或低出生体重儿影响的指标, 包括高龄产妇、婴儿性别、高龄父亲年龄以及一组社会经济状况（SES）这些变量。然后我们收集高龄产妇年龄系数并绘制其趋势图。我们在4,153,313名第二次及第三次分娩的人中重复此程序。

结果：当我们只控制婴儿性别时, 高龄产妇与早产之间的关系在21世纪00年代年急剧减弱, 此后略有增加：与20世纪90年代后期高龄产妇的早产增长至3.5％有关, 但到21世纪10年代下降到不足2％。控制高龄父亲年龄后轻微减弱此关系, 控制SES后几乎不影响此关系。关于低体重出生儿我们也得出同样的结论。第二次及第三次分娩展示出高龄产妇对妊娠结局更小、更弱的影响。

结论：相对而言, 高龄产妇比年轻产妇更可能发生早产及低出生体重儿。然而, 绝对而言, 在20世纪90年代该风险是小的, 在21世纪10年代早期该风险更小。

Keywords:

• Maternal age
• preterm birth
• low birthweight
• South Korea

Acknowledgements

I am grateful to the anonymous reviewer and Irving Sivin for helpful comments and suggestions.

Disclosure statement

There is no conflict of interest.

Notes

1 Detailed data are available at: https://data.oecd.org/pop/fertility-rates.htm.

2 Gestational length in the hospital report was determined by either the last menstrual period or an ultrasound exam, the latter being quite accurate. The data did not indicate which method was used, but indirect evidence suggests that the length was accurate. We analysed publicly available data on all infant deaths between 2009 and 2014, which contained information as to whether the mother received prenatal care. Prenatal care in Korea involves ultrasound exams, and if it does not adversely affect birth outcomes (the opposite is the intention), this information should provide lower bounds of percentages of mothers who received prenatal care for all births. Table A-1 in the Supplementary appendix shows that most received prenatal care, implying that hospitals provide accurate gestational lengths for most births.

3 Slightly varying the cut-off points for advanced maternal and paternal ages did not change the substance of our results (not shown).

4 Applying a probit or logit regression did not change the substance of our results (not shown).