At a Glance: Choosing the Best Sibling Age Gap
- Parents can use research to inform their decisions but cannot always control the age gap between siblings.
- An age gap of 27 to 32 months may be associated with the best health outcomes for mother and child.
- Smaller age gaps are associated with more sibling fighting, as well as more parent stress.
- Both very and very large gaps may correspond to an increased risk for neurodevelopmental disorders.
- Larger sibling age gaps are associated with higher levels of school readiness and higher grades.
- The “best” sibling age gap is a personal decision that depends on your family, finances, and health.
Why “Best” Age Is Different for Every Family
Let’s get the most important part out of the way first: there is no “best” age that works for every family. That’s because every family is different. The right age gap is the one that works for you.
Plus, “deciding” what age gap your kids will have is, in many ways, impossible. As with so many things parenting, we can plan as much as we want. But age, fertility, financial constraints, and other factors can still get in the way.
Even when parents do have a choice, the research can only provide so much insight. And it doesn’t suggest a definitive “ideal” sibling age gap. Ultimately, your personal preferences and goals are likely more important than any research study.
However, deciding when to add a new child to the family is a difficult decision for many parents. My hope is that knowing the research on this topic can help parents who feel “stuck” in this decision. To help you out, I’ve rounded up and summarized the research on the benefits and risks of different age gaps.
How Does Sibling Age Gap Impact Physical Health?
First, let’s look at how the age gap between siblings may impact the physical health of the children and the mother.
Impact on Babies
A meta-analysis found that there may be negative health outcomes for children when there is less than six months or more than five years between pregnancies. This means children who are less than 15 months or more than 5 years and 9 months apart in age.
A meta-analysis is a study that combines data from many previous studies on a topic.
These negative health outcomes may include:
- Prematurity: Being born before the start of the 37th week of pregnancy
- Low birthweight: An infant born weighing less than 5.5 pounds
- Being small for gestational age: A baby that weighs less than expected based on when they were born
More recent research found additional information. Less than six 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 like the U.S., Australia, Norway, or Finland.
However, there was still increased risk for births that happened before the 37th week of pregnancy without induction.
Impact on Mothers
The age gap between children is also related to potential health risks for the mother during pregnancy and labor. Research finds that:
- More than five years between pregnancies is associated with an increased risk for preeclampsia.
- Less than two years between pregnancies is associated with delayed or slow labor.
- Less than 12 months between pregnancies may increase the risk for placental abruption, when the placenta detaches from the uterus.
Another finding was 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. This means your children would be 27 to 32 months apart in age.
Understanding Why
Now, this research doesn’t always tell us why this different risks happen. But researchers speculate there may be a few reasons.
When it comes to maternal health, conceiving again shortly after being pregnant may be associated with other risk factors. For example, mothers who do not regularly seek healthcare may be more likely to get pregnant again quickly. And not getting regular healthcare could make you 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. This is sometimes called the maternal depletion hypothesis. If your body is still worn out from your first pregnancy, the second one may come with increased health risks.
What about longer age gaps? Researchers speculate that a mother’s body is primed for pregnancy after having one baby but gradually loses this after giving birth. This is sometimes called the physical regression hypothesis. Or, a longer age gap may happen because of a health risk factor that causes both fertility issues and a greater chance of complications during pregnancy and delivery.
How Does Sibling Age Gap Impact Developmental Outcomes?
How might the age gap between siblings impact the development of the child?
Language, Communication & Math
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. However, this study found no difference for younger siblings.
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. Some studies find that even children born more than 3.5 years apart show lower math scores.
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.
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.
Educational Attainment
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.
Neurodevelopment Disorders
Research also finds that very small age gaps and very large gaps may both correspond to 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. It also found that 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. It also found that pregnancies that are more than 10 years apart are associated with a 25% increased risk for ADHD.
Understanding Why
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. This could impact both age gap and how they parent. Or, parents may decide to wait longer to have a second child if the first child has a developmental delay or a disability like ADHD.
When it comes to language development and educational outcomes, parents have more time to talk, play, and read with children when there is a larger age gap. This improves communication and developmental outcomes for both children. For example, some research finds that parents read more to older children when there is a larger age gap between siblings.
How Does Sibling Age Gap Impact Sibling Relationship Quality?
What about how kids interact with each other? How does the age gap between siblings impact the quality of their relationship?
Research finds that wider age gaps between siblings seem to be related to less conflict. Smaller age gaps, on the other hand, are related to a closer sibling relationship.
Specifically, siblings aged 4 or more years apart may also show greater affection, prosocial or kind behavior, and admiration towards one another. 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.
How Does Sibling Age Gap Impact 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 24–49% 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. However, they also may be related to other differences between families who choose to have siblings closer together and families that do not.
Limitations of this Research
It’s important to remember that these studies are all correlational. This means they only find an association between age gaps and outcomes. They don’t tell us that age gaps actually cause any of these things.
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. These factors can’t always be measured and controlled for in a research study. For example, factors like exclusive breastfeeding and regularly seeing a doctor may result in a bigger age gap. And seeing a doctor more could also correspond to better health for mothers and babies.
In order to determine whether an age gap causes positive or negative outcomes, researchers would have to randomly assign parents to different age gaps. That’s both practically and ethically impossible for so many reasons!
Another problem with this research is that it doesn’t measure many of the things that are important to families. For example, you might want to have children less than 2 years apart in age so they can play together more. Or maybe you want to give each child more one-on-one time when they’re younger, which is easier with a bigger age gap. Plus, the age gap between siblings is often outside of our control!
However, in spite of these limitations, I still think it can be important to discuss this research. These correlational studies are the best data we have on the impact of age gap. (And it is likely to stay that way).
Making the Decision: When Should You Have Another Baby?
Ultimately, when to have another baby is a personal choice. Research can tell you about general outcomes and possibilities. But it can’t tell you what’s best for your family’s health, finances, or emotions.
Your parenting is going to matter more for your kids than how far apart they are in age. And if you look around, I’m sure you’ll spot happy, thriving families with every possible age gap between siblings.
Sources & Citations
Find links to all of the research studies cited here.
Get more of the latest research on families and parenting explained by an expert, Learn how to deal with sibling rivalry, science-backed ways to manage parent stress, and why doctors love to see kids playing in nature.
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DR. CARA DAMIANO GOODWIN, PhD, is a licensed clinical psychologist and a Charlottesville mother to four children. She received a PhD in child clinical psychology from the University of North Carolina at Chapel Hill, a Master’s in Developmental Psychiatry from Cambridge University, and a Master’s in Child Psychology from Vanderbilt University. Dr. Goodwin translates recent scientific research into information parents can access and implement in their everyday lives through her Instagram account @parentingtranslator and her website ParentingTranslator.com.