What is the best age gap between siblings?

The research behind how the age gap between siblings relates to physical and mental health, the sibling relationship, and parent stress

Dr. Cara Goodwin, PhD

Mar 1, 2023

Source: Pexels/Victoria Akvarel

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One of the most common questions I get through my Parenting Translator platform is: What is the ideal age gap between siblings? I have been hesitant to address this topic because many parents do not have the privilege of making this decision due to age, fertility, financial constraints, and/or other factors. Even when parents do have a choice, the research can only provide so much insight because your own personal preferences and goals for your family are likely more important than any research study.

Even as someone who is undeniably obsessed with the research, I didn’t consider the research at all when I was making this decision for my own family. Instead, my decision was based on the age that I started having children and my desire to have a large family (in order to achieve that goal I decided that I should not have age gaps that were longer than 3 years). I also wanted to emulate my own family growing up. I am the middle of three children and we are all 18 months to 2 years apart and, although there was a lot of fighting, there was also a lot of closeness and love. I was fortunate to not have fertility problems and my children are all 2 to 3 years apart. I have generally been happy with these age gaps but, of course, I see families with children farther apart in age who never fight or families with “Irish twins” who have children at nearly the same developmental stage, and I can’t help but think that the grass is greener on the other side of the “age gap fence.”

So although I cannot emphasize enough that the best age gap is undoubtedly the one that works best for your individual family, I also realize that deciding when to add a new child to the family is a very difficult decision and knowing the research on this topic may help parents who feel “stuck” in this decision. So if you feel like you do need some guidance, this newsletter summarizes the research on the benefits and risks of different age gaps.

Major Problems with this Research

Even before diving into the research, it is very important to discuss the limitations of this research. These studies are all correlational, meaning they only find an association between age gaps and outcomes and we have no evidence that age gaps actually cause any of these outcomes. Families who choose to have children closer together in age are different from families who choose to have children farther apart in age in many different ways that cannot always be measured and controlled for in a research study. Therefore, any one of these factors may be the true cause of these differences rather than the age gap itself. For example, factors like exclusive breastfeeding and regularly seeing a doctor may result in an increased age gap between siblings. A factor like being very responsive to your child or a more careful decision maker may result in a larger age gap.

Another problem with this research is that a lot of factors that really matter to families are not measured by the research. For example, if you have children that are less than 2 years apart of age, they may play together more or parenting may be easier because you can teach them the same concept at the same time and they are more likely to be interested in doing the same activity or reading the same book with you. For children with age gaps larger than 5 years, the older child is likely to be more helpful with the younger child thus reducing your overall stress as a parent.

However, even given these very serious limitations, I still think it might be important to discuss this research because these correlational studies are the best data we have on the impact of age gap (and it is likely to stay that way). In order to determine whether age gap causes positive or negative outcomes, researchers would have to randomly assign parents to different age gaps which is practically and ethically impossible for so many reasons.

Physical Health

First, it is important to discuss how the age gap between siblings may impact the physical health of the children and the mother. A meta-analysis (meaning a study that combines data from all previous studies) found that there may be negative health outcomes for children when there is less than 6 months or more than 5 years between pregnancies (meaning children who are less than 15 months or more than 5 years and 9 months apart in age). These negative health outcomes may include prematurity (translation: being born before the start of the 37th week of pregnancy), low birthweight (translation: an infant born weighing less than 5.5 pounds), and being small for gestational age (translation: a baby that weighs less than would be expected based on the weeks of pregnancy at which the baby was born). However, more recent research found that less than 6 months between pregnancies (which means less than 15 months between children) was not associated with an increased risk for low birthweight and small for gestational age babies in higher income countries (such as the US, Australia, Norway, Finland). However, there was still increased risk for spontaneous preterm births (translation: births that occur before the 37th week of pregnancy in which labor is not induced but starts spontaneously).

The age gap between children is also related to potential health risks for the mother during pregnancy and labor. Research finds that longer ages between pregnancies (more than 5 years) are associated with an increased risk for preeclampsia (translation: a condition during pregnancy involving high blood pressure). Less than two years between pregnancies is associated with labor dystocia (translation: delayed or slow labor) and women with less than 12 months between pregnancies show an increased risk for placental abruption (translation: when the placenta detaches from the uterus).

In addition, research finds that the age gap between children may be related to risk of childhood injury. Specifically, research finds that the number of injuries requiring hospitalization for children increases when the age gap is less than 24 months or greater than 48 months.

Based on this body of research, a recent study concluded that the optimal length between pregnancies for the best health outcomes may be 18 to 23 months, which means your children would be 27 to 32 months apart in age. Why might this be? Researchers speculate there may be a few reasons. First, choosing to conceive (or choosing not to prevent conception) shortly after being pregnant may be associated with other risk factors. For example, mothers who do not regularly seek health care may be more likely to conceive quickly after a previous pregnancy and more likely to have a baby with negative health outcomes like prematurity. Another explanation is that when pregnancies are close together the mother’s body does not have the chance to fully recover from the stress of pregnancy and lactation (referred to as the maternal depletion hypothesis). What about longer age gaps? Researchers speculate that longer intervals between pregnancies may have negative impacts because the mother’s body is primed for pregnancy after having one baby and the mother gradually loses this status after giving birth (referred to as the physical regression hypothesis). In addition, mothers who don’t get pregnant for many years after their previous child may have other health risk factors that cause both fertility issues and a greater likelihood of complications during pregnancy and delivery.

Developmental Outcomes

How might the age gap between siblings impact the development of the child? Research finds that older siblings who are more than 2 years apart from their younger sibling show higher test scores in reading and math than children who are less than 2 years apart from their younger sibling (yet this study found no difference for younger siblings). Other research finds that, when siblings are less than 21 months apart, the younger child scores lower on vocabulary measures and lower math and reading scores. Even in high school, children with siblings less than two years apart scored lower on a test of verbal IQ than siblings closer in age.

There may also be negative outcomes associated with larger age gaps. Research finds that children who are more than 5 years apart also show lower scores on a communication measure and even children born more than 3.5 years apart show lower math scores.

However, it is also important to mention that more recent research finds no relationship between age gap and language development. This study instead found that having an older sibling enhanced children’s language skills regardless of age gap.

Age spacing also seems to impact education. Closely spaced siblings are less likely to complete high school and attend college. In particular, a sibling age difference of 2 years or more was associated with a higher chance of completing high school and attending college. Another study found that children who are more than 3.5 years apart tend to get higher grades than children born less than 2 years apart. Finally, research finds that children with less than a 24 month age gap show lower levels of school readiness.

Research also finds that there may also be an increased risk for neurodevelopmental disorders. For example, a review study found that pregnancies that are less than 12 months apart (meaning siblings that are less than 21 months apart) are associated with a nearly 2 times increased risk for autism while pregnancies that are more than 5 years apart are associated with a 30% increased risk for autism. Another study found that pregnancies that are less than 6 months apart are associated with a 30% increased risk for ADHD and pregnancies that are more than 10 years apart are associated with a 25% increased risk for ADHD.

Why might sibling age gap impact child development? It may be the maternal depletion hypothesis or the physical regression hypothesis described above. Another very likely possibility is that parents who choose to have children closer together in age are different in other ways (such as being more impulsive) which may impact both age gap and parenting. In addition, parents who choose to have children far apart in age are different from parents who did not. For example, they may have had a more negative experience with their first child, which impacted both how they parented their second child and when they chose to have a second child. A final possibility is that parents simply have more time to talk, play, and read with children when there is a larger age gap which improves developmental outcomes. For example, there is some research finding that parents read more to older children when there is a larger age gap.

Sibling Relationship Quality

What about the sibling relationship? How does the age gap between siblings impact the quality of the sibling relationship? Research finds that wider age gaps between siblings seem to be related to less conflict, while smaller age gaps are related to a closer sibling relationship (Newman, 1996). Specifically, siblings aged 4 or more years apart may also show greater affection, prosocial or kind behavior, and admiration towards one another, while siblings aged less than 4 years apart are more likely to be close. Research also finds that greater conflict among siblings closer in age persists into adulthood. However, age gap seems to matter most in early and middle childhood and less in adolescence.

Parenting Stress

Research finds that when parents have children that are closer in age, they are more likely to be stressed. Parents with children less than 18 months apart are also more likely to take prescription medication and more likely to die early in middle age or early old age compared to parents with children 30 to 41 months apart. In addition, having children closer together in age may be linked with an increased risk for divorce. Parents who had children 1.5 years or less apart in age had a 24 to 49 percent higher risk of divorce than parents with children 4 or more years apart. These effects may be related to the greater demands on parents when having children close in age but they also may be related to other differences between families who choose to have siblings closer together and families that do not.

Overall Translation

Across many different studies, a shorter age gap or a longer age gap is associated with worse health and developmental outcomes. Research suggests that an age gap of 27 to 32 months may be associated with the best health outcomes for mother and child. In terms of sibling relationship, siblings who are closer in age tend to fight more but are also closer which may have positive benefits for social skills. Parents who have children closer in age may be more stressed and more likely to have relationship problems. However, there are major problems with this research that makes it difficult to interpret and does not suggest a clear “ideal age gap.” Parents who are fortunate enough to have the privilege of making a decision about age gap between children should make this decision based on what is best for their family.

Expert Review

All Parenting Translator are reviewed by experts in the topic to make sure that they are as helpful and as accurate for parents as possible. Today’s newsletter was reviewed by Rebecca Berlin, PhD. Dr. Berlin received her PhD from the University of Virginia School of Education and has served as a special education teacher, home visitor, child assessor, autism specialist and school administrator. She has conducted research on the teacher-child interactions, as well as play and story based interventions for improving social skills and classroom quality. She currently serves as the Executive Director for the Parenting Translator Foundation. She is an only child and the mother of teenagers with a 44 month age gap.

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