What’s the Ideal Age Gap Between Siblings

To the extent to which one can plan for it, is there such a thing such as a best age difference between two siblings? “Best” as in both parents and children have the best possible experiences going forward. While the answer is unlikely to be a single number, it is worth looking for a range of preferrable age gaps based on scientific research.

Sibling age gap and its link to parental stress and health

Arguably, parenting stress is a common denominator for all kinds of outcomes for children and the family unit as a whole. The less stressed the parents are, the better everything else should be. While there is not a lot of research on the topic, one recent study ^[1] focused on mother’s stress levels by studying over 900 families, around 300 of which were single child families and served as the control group. It found that within the two-child families, the greater the age gap was, the less stressed the mother was. However, it should be noted that the effect was quite small, explaining a bit over 1.5% of the overall difference in stress between mothers of a single child versus those parenting two children.

Another study ^[2] followed parents over a ten-year period and examined the relationship between sibling age difference and parental divorce rates, found that shorter birth intervals are associated with increased risk of divorce. In particular, if the two children were born no more than 1.5 years apart, the associate divorce risk is 49 per cent higher when compared to individuals with two children born more than 4 years apart.

A 2014 study of the entire Norwegian population ^[3] showed that that mothers and fathers of two to three children with intervals between births of less than 18 months (1.5 year age gap), as well as mothers of twins, had raised mortality risks in midlife and early old age relative to parents with interbirth intervals of 30–41 months (2.5-3.5 year gap). A small age difference between first and second born was found to be associated with higher use of medications. On the other hand, very large age gaps were not linked to raised mortality or medication use.

Based on the above, it is safe to say that if you can choose, it is best to space births out by at least 2.5 to 3 years. A sibling age gap of 3.5 years or more seems preferrable in terms of minimizing adverse long-term implications for parental health, parental stress and the likelihood of divorce. Waiting for too long, however, is associated with certain complications surrounding pregnancy and delivery ^[4] , so waiting for too long is not advisable either. Using an age gap calculator can help you figure out what age difference you can achieve.

Siblings age difference and outcomes for the children

We talked about parents, but for most people what’s best for children comes first. Well, in this case you wouldn’t have to choose since it seems the literature largely shows that what is best for mommy and daddy is best for the children as well.

For example, a large Australian study published in the British Medical Journal ^[5] shows that children born more than 6 months apart but less than 24 months apart, show the best outcomes across a wide range of measures. These include physical health and wellbeing, social competence, emotional maturity, language and cognitive skills (school-based) and communication skills and general knowledge.

The paper discusses the well-established J-shaped relationship between how many months apart the births are, with too few being associated with worse outcomes, an “ideal” zone at the bottom of the J-shaped curve, and then increasingly higher risks as the number of years apart grows from four to five years and more.

Other works also support this in different narrower domains. For example, a paper ^[6] in the journal Injury Prevention concluded that while the presence of any siblings increases the risk of physical injury on a child, those who had siblings with very short birth intervals had the highest risk of injury. In particular, an age gap of less than two years showed the greatest increase in relative risk. As for the reasons, the authors speculate that it may be due to the inadequate parental supervision caused by the demands of caring for two children of very similar age.

Another scientific study focuses on the effects of birth spacing on IQ and academic performance ^[7]. It finds beneficial effects of a larger age gap on the older siblings, with large negative effects associated with spacing of less than two years. No negative or positive effects were observed for the younger siblings. The paper also states that birth spacing can be an important way for parents to improve child outcomes when it comes to academic achievements.

The ideal age gap between siblings

The World Health Organization (WHO) recommends ^[5] an age difference between siblings of 2 to 3 years, based on a variety of studies focused on child outcomes during and shortly after birth. The studies presented above confirm that an age gap of over two years, but less than five years, seems ideal for both parents and children, across a wide range of measures some going well into the future of both.

While it may not be possible for everyone to achieve such birth spacing, it is something to try and plan for, to the extent such things are in our hands as would-be parents. The raised awareness of the relatively elevated risks associated with births that are too few months apart, as well as those that are too many years apart, may be of help to those of you trying to plan their family.

Importantly, none of the above should be viewed as a recommendation to avoid a pregnancy if it happens to fall outside of these ideal boundaries, nor is it raise undue alarm about the well-being of siblings who do not fall into that age gap range.

Cited scientific literature:

[1] Qian G, Mei J, Tian L and Dou G (2021) Assessing Mothers’ Parenting Stress: Differences Between One- and Two-Child Families in China. Front. Psychol. 11:609715.

[2] Berg V, Miettinen A, Jokela M, Rotkirch A (2020) Shorter birth intervals between siblings are associated with increased risk of parental divorce. PLOS ONE 15(1): e0228237.

[3] Grundy E, Kravdal Ø (2014) Do short birth intervals have long-term implications for parental health? Results from analyses of complete cohort Norwegian register data. J Epidemiol Community Health; 68:958-964.

[4] Conde-Agudelo A, Rosas-Bermúdez A, Kafury-Goeta A. (2007) Effects of birth spacing on maternal health: a systematic review, American Journal of Obstetrics & Gynecology, 196, 297-308

[5] Dhamrait GK, Taylor CL, Pereira G. (2021) Interpregnancy intervals and child development at age 5: a population data linkage study. BMJ Open;11:e045319

[6] Nathens AB, Neff MJ, Goss CH, et al. (2000) Effect of an older sibling and birth interval on the risk of childhood injury. Injury Prevention;6:219-222.

[7] Buckles, K. S., & Munnich, E. L. (2012). Birth Spacing and Sibling Outcomes. The Journal of Human Resources, 47(3), 613–642. http://www.jstor.org/stable/23214405