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Birth order and health of newborns

What can we learn from Danish registry data?


We examine birth order differences in health of newborns and follow the children throughout childhood using high-quality administrative data on individuals born in Denmark between 1981 and 2010. Family fixed effects models show a positive and robust effect of birth order on health at birth; firstborn children are less healthy at birth. During earlier pregnancies, women are more likely to smoke, receive more prenatal care, and are more likely to suffer a medical pregnancy complication, suggesting worse maternal health. We further show that the health disadvantage of firstborns persists in the first years of life, disappears by age seven, and becomes a health advantage in adolescence. In contrast, later-born children are throughout childhood more likely to suffer an injury. The results on health in adolescence are consistent with previous evidence of a firstborn advantage in education and with the hypothesis that postnatal investments differ between first- and later-born children.

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  1. See, for instance, Price (2008), Lehmann et al. (2017), Hotz and Pantano (2015), Pavan (2015).

  2. See, for instance, Behrman and Taubman (1986), Ejrnæs and Pörtner (2004), Hotz and Pantano (2015).

  3. In line with this literature, we observe a negative effect of birth order on ninth grade GPA, see Appendix Table 7.

  4. For references to the medical literature, see Camilleri and Cremona (1970), Magnus and Bjerkedal (1985), Swamy et al. (2012), Hinkle et al. (2014).

  5. Missing information on health at birth results from (1) unrecorded data, which occasionally happens at the beginning of the data in 1981, (2) biological implausible values, and (3) non-existing information due to perinatal child death. When we look at the prenatal environment, we include information also for those children with missing birth outcomes to eliminate problems of selection.

  6. Our results also hold for families with five to eight children.

  7. We have fewer observations for birth order one due to a larger number of birth outcomes with missing information in the birth records, see footnote (5).

  8. The 5-minute Apgar score is a diagnostic test measured 5 min after birth and based on five criteria: heart rate, respiratory effort, muscle tone, reflex irritability, and color. For each criteria zero, one, or two points are assigned with the resulting score ranging between zero and ten. The Apgar score has been found to be highly correlated with cognitive ability, health, and behavioral problems in later childhood (Almond et al. 2005). Considering the 1-minute Apgar score (measured 1 min after birth) instead of the 5-minute Apgar score provides very comparable results. However, as the data does not provide us with the 1-minute Apgar score after 1996, we focus on the 5-minute Apgar score only.

  9. Using the first component of a principal component analysis of birth weight, low birth weight, high birth weight, prematurity, and Apgar score yields very similar results.

  10. Unfortunately, we do not observe alcohol consumption during pregnancy and are therefore unable to analyze this aspect of maternal behavior.

  11. Diagnoses are based on the International Statistical Classification of Diseases and Related Health Problems, 8th and 10th Revisions (ICD-8 and ICD-10). The reporting standard changed in 1994 from ICD-8 to the ICD-10 codes. However, we can still use information for all diagnoses in our sample, using the recoding of the old ICD-8 codes from Lykke et al. (2012) to merge with the ICD-10 codes.

  12. Casey et al. (1997) report that between 1 and 3% of all pregnancies in the USA are diagnosed with gestational diabetes.

  13. If women experiencing preeclampsia are counted, the figure for gestational hypertension would increase to 4%. Sibai (2003) notes a prevalence of gestational hypertension in the USA of 6 to 17% for nulliparous women and 2 to 4% for multiparous women. These numbers fit in line with the 4% given that we have 43% of nulliparous births and 57% of multiparous births in our sample.

  14. We prefer the use of month of conception and year of conception over the use of month of birth and year of birth to compare children that have the same expected conditions in utero. This is in line with Persson and Rossin-Slater (2016), Almond and Mazumder (2011). However, our results do not change when we substitute month of conception and year of conception with month of birth and year of birth, respectively.

  15. Except for the difference between birth order three and birth order four in column (4), the increase in the coefficients for each additional birth order is significant at the 1% level.

  16. Additional measures for health at birth include the natural logarithm of birth weight and birth length, birth weight z-score, small for gestational age (SGA), large for gestational age (LGA), head circumference (available since 1997), and an indicator for being diagnosed for a condition relating to the perinatal period (available since 1994).

  17. Perinatal death is defined as fetal deaths occurring with a stated or presumed gestation of 28 weeks or more or deaths occurring within the first 7 days of life. These children are grouped on the assumption that similar factors have caused the death (Barfield 2011). The definition is furthermore the official definition for perinatal death used by the National Center for Health Statistic and the World Health Organization. Notice that we have more observations for perinatal deaths than for our other health outcomes, as not all children dying in the perinatal period have information on these other outcomes.

  18. The reason for this cut-off is that we observe births through December 31, 2010. This is a reasonable cut-off, as 91% of all women who were above 45 years in 2010 got their last child before the age of 38. This restriction decreases modestly age at birth by roughly half a year and decreases the interpregnancy interval by 1.5 months.

  19. As the relationship in Table 4 column (6) is weaker than in column (2), these results indicate that some women quit smoking during pregnancy. Restricting the sample to women who smoked anytime in their first pregnancy, we continue to find that birth order has a negative effect on smoking at the end of pregnancy. We attempted to replicate the results from Black et al. (2016b) who study the probability to stop smoking conditional on smoking at the beginning of any pregnancy. To be able to estimate a family fixed effects model, this requires at least two observations within a family where the mother smoked at the beginning of the pregnancy (anytime during the pregnancy) and a variation in the probability to still smoke at the end. Even though this creates a very selected sample, we did not find that the order of the pregnancy (birth order) relates to the probability to stop smoking during pregnancy.

  20. Because information on both prenatal smoking and prenatal care is available only for a subsample of all children, we test the robustness of our findings towards estimating the effect of birth order on child health at birth for the sample of children for which we have this information (common sample). Our results are robust to this exercise.

  21. Due to the restriction of the hospital admission data, we do not observe the oldest cohorts when they are young. For example, the cohort born in 1981 will be observed from age 13 onwards.

  22. We also exclude conditions related to pregnancy and childbirth as well as congenital malformations and conditions originating from the perinatal period to not confuse this analysis with the health at birth analysis. We include chapters I) Certain infections and parasitic diseases, II) Neoplasms, III) Diseases of the blood and blood-forming organs and certain diseases, IV) Endocrine, nutritional and metabolic diseases, V) Mental and behavioral disorders, VI) Diseases of the nervous system, VII) Diseases of the eye and adnexa, VIII) Diseases of the ear and mastoid process, IX) Diseases of the circulatory system, X) Diseases of the respiratory system, XI) Diseases of the digestive system, XII) Diseases of the skin and subcutaneous tissue, XIII) Diseases of the musculoskeletal system and connective tissue, and XIV) Diseases of the genitourinary system.

  23. We find similar results when considering emergency room contacts instead of hospitalizations for injuries, underlining that the health disadvantage of later-borns stems from acute rather than chronic diseases.

  24. Lundberg and Svaleryd (2017) group children into larger age groups and focus on inpatient contacts to the hospital only. While the authors also study health differences in early childhood, they do not find a robust firstborn advantage in hospitalizations before age six.


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We are thankful for helpful comments and suggestions from three anonymous referees, Stefan Bauernschuster, Sanni Nørgaard Breining, Michael Grimm, Timo Hener, Edward Samuel Jones, Jacob Alexander Lykke, Torben Heien Nielsen, Helmut Rainer, Heather Royer, members of the University of California Santa Barbara Human Capital Working Group, and participants at the Annual Meeting of the Society of Labor Economists 2016, the Royal Economic Society Conference 2016, the BGPE Research Workshop 2016, the Essen Health Conference 2015, the Bavarian Micro Day 2015, seminars at University of California, Santa Barbara; the University of Copenhagen; the Danish National Centre for Social Research; and the University of Passau.

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Correspondence to Anne Ardila Brenøe.

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Responsible editor: Erdal Tekin


Appendix A: The nature hypothesis

The general understanding in the medical literature is that physiological changes during the first pregnancy, necessary for fetal development, do not fully return to their baseline value (before the first pregnancy). Higher order pregnancies profit from this incomplete reversal. These physiological changes encompass the uterine blood supply (Hafner et al. 2000; Hollis et al. 2003; Khong et al. 2003; Prefumo et al. 2004) and an enlargement of the uterus (Woessner and Brewer 1963; Sønes and Bakke 1989), both of which affect nutrient supply to the fetus (Gluckman and Hanson 2004). It has also been suggested that maternal sensitization to paternal antigens that occur at the first pregnancy affect birth weight of later-born children (Warburton and Naylor 1971; Chakraborty et al. 1975).

Animal studies perpetuate the findings from the medical literature. A positive effect of birth order on health at birth appears for cattle (Johanson and Berger 2003) as well as sheep (Gardner et al. 2007). Animal studies occur in a controlled environment, for example, with respect to nutrition, and therefore alleviate concerns about endogenous behavioral differences of the mother.

Appendix B

Fig. 4
figure 4

The effect of birth order on health at birth by maternal age at first birth. Notes: The figure plots the coefficients of the interaction term between birth order and maternal age at first birth in the family fixed effects model (model (1) where the three birth order dummies are interacted with five dummies for maternal age at first birth). The dependent variable is the health index (mean of zero and standard deviation of one) that is an equally weighted summary index of the following variables: birth weight, low birth weight, high birth weight, prematurity, and Apgar score. Age at first birth is divided into (1) < 22 years, (2) 22–25 years, (3) 26–29 years, (4) 30–33 years, (5) > 33 years. The regression includes family fixed effects, interactions between year of conception and month of conception dummies, and a dummy for gender of the child. The whiskers indicate the 95% confidence interval

Fig. 5
figure 5

The effect of birth order on health at birth by mother’s highest education. Notes: The figure plots the coefficients of the interaction term between birth order and education of the mother in the family fixed effects model (model (1) where the three birth order dummies are interacted with three dummies for mother’s highest education). The dependent variable is the health index (mean of zero and standard deviation of one) that is an equally weighted summary index of the following variables: birth weight, low birth weight, high birth weight, prematurity, and Apgar score. Education is divided into (1) < HS: no high school/education (< 12 years), (2) HS: high school and potentially some vocational training or two years of college, and (3) BA: Bachelor degree or more. The regression includes family fixed effects, interactions between year of conception and month of conception dummies, and a dummy for gender of the child. The whiskers indicate the 95% confidence interval

Fig. 6
figure 6

The effect of birth order on health at birth by gender of the child. Notes: The figure plots the coefficients of the interaction term between birth order and gender of the child (model (1) where the three birth order dummies are interacted with a dummy for boy and a dummy for girl). The dependent variable is the health index (mean of zero and standard deviation of one) that is an equally weighted summary index of the following variables: birth weight, low birth weight, high birth weight, prematurity, and Apgar score. The regression includes family fixed effects, interactions between year of conception and month of conception dummies, and a dummy for gender of the child. The whiskers indicate the 95% confidence interval

Table 7 Effect of birth order on ninth grade GPA
Table 8 Effect of birth order on child health at birth —additional measures
Table 9 Effect of birth order on health throughout childhood

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Brenøe, A., Molitor, R. Birth order and health of newborns. J Popul Econ 31, 363–395 (2018).

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  • Birth order
  • Child health
  • Fetal health
  • Health at birth
  • Prenatal investments

JEL Classification

  • I10
  • I12
  • I14
  • J12
  • J13