Child Spacing: What’s the Best Age Gap for Siblings?
As you plan your family, you may be wondering how far apart to space your children and whether this is an optimal age gap. In this article, we examine data on average spacing, health perspectives, and benefits of more or less time between children.
Whether you are trying to conceive, already have a baby and are thinking about a sibling, are in the process of adopting, or are simply envisioning your future family, you may be wondering how far apart to space your children.
With a 31-month age gap between myself and my younger sister, I always just assumed that targeting a two to three-year age gap between my own kids someday would make sense. But what is optimal for mom and baby’s health? What about sibling relationships? And what other factors should be taken into consideration?
How My Parents Approached Child Spacing
I’m often reminded that August and September leading up to my birth were unseasonably hot, and my parents didn’t have air conditioning at the time. Understandably, being eight to nine months pregnant during an extended heat wave was not exactly my mom’s idea of a great time.
When trying to conceive her second child, my mom—the most diligent planner I know—decided that if she wasn’t pregnant by the end of July, they would wait another year to try again. Even though she was hoping for less than three years between children, she wanted the timing to line up with a winter pregnancy and spring birth. Luckily the timing worked out, and my sister was born the following April.
While, unfortunately, there are no guarantees with timing no matter how much you plan, this story and many others got me thinking about what circumstances my fiancé and I should consider as we start planning for children ourselves over the next few years.
Average Age Gap Between Siblings
First, let’s cover some nomenclature. The interpregnancy interval is defined as the time between pregnancies, regardless of the pregnancy outcome (i.e., the time in between a miscarriage and subsequent pregnancy, or time between a live birth and subsequent pregnancy).  The birth-to-birth interval, however, is the time between live births.
The Centers for Disease Control and Prevention (CDC) released a report in 2015 comparing birth-to-birth intervals in the United States (US) based on analysis of birth certificates and self-reporting through the National Survey of Family Growth.  They found that ~30% of births in the US from 2006-2010 were less than 18 months apart, ~50% were 18-60 months apart, and ~20% were greater than 60 months apart. While this doesn’t specifically address the question of exactly how far apart pregnancies are spaced, it does tell us that ~80% of births are less than five years apart for women with multiple children (with most of those being between one-and-a-half and five years apart). 
The CDC did find that time between births varied by state. On the shorter end, they observed a median birth-to-birth interval of 25 months in some states (e.g., Idaho, Montana), and on the higher end, a median interval of 32 months in California. 
Best Age Gap for Siblings From a Medical Perspective
The American College of Obstetrics and Gynecology (ACOG) recommends that women avoid intervals of under six months between giving birth and the start of the next pregnancy, and that they seek counseling from their medical provider about the risks of interpregnancy intervals less than 18 months. 
ACOG recognizes that although the literature suggests interpregnancy intervals of less than 18 months or greater than 60 months are associated with increased risk of adverse outcomes such as low birth weight, those risks may not actually be causal (i.e., don’t take into effect socioeconomic status or maternal age).  For example, a recent study in Canada compared birth outcomes from the same set of data on a generalized basis (across all subjects) and a within-woman basis (matching outcomes to each woman).  While the generalized analysis showed higher rates of negative outcomes such as preterm births, when they looked at the data on a per-woman basis, those risks were no longer apparent.  This would suggest that short interpregnancy intervals may not have a causal relationship with higher risks (even though they are correlated across subjects, likely due to other factors confounding the data). However, they also found higher rates of gestational diabetes among women with interpregnancy intervals less than five months even with the adjusted analysis, suggesting further studies need to be conducted to better understand the risks for moms and babies alike. 
Another recent study in Sweden looked at longitudinal data and found that birth intervals have little independent effect on long-term health outcomes for children as they grow up.  Between this study and the Canadian study, it seems you can focus mostly on the maternal health implications as you think through your decision.
For example, there are some specific considerations for births subsequent to cesarean section delivery. For women who have cesarean deliveries and then attempt vaginal delivery for a subsequent birth, interpregnancy intervals of less than 18 months are associated with higher rates of uterine rupture during delivery. 
For women who have experienced a miscarriage, recent studies suggest that short interpregnancy intervals (even less than three months after pregnancy loss) are not associated with a greater risk of subsequent miscarriages.  Furthermore, a study from 2017 even showed that an interval of less than three months after a miscarriage before conceiving again was associated with the lowest risk of subsequent miscarriage.  This would mean that after a miscarriage, once you get the green light from your doctor to try again, you may not need to delay trying to conceive against an artificial three or six-month timeline. For more information on conceiving after miscarriage, check out this article on getting pregnant after miscarriage by Dr. Kenosha Gleaton, OBGYN.
With the average maternal age at first birth increasing over recent years, you may be wondering how maternal age factors into the decision to space children closer or farther apart. For those of you who have read Dr. Nazaneen Homaifar’s article on how age affects fertility, you might remember that while a healthy woman in her 20s or early 30s has a 25%-30% chance of conceiving each month, the chances are about ten percent (or less) each month by the time a woman is 40 years old. Given that it is biologically more difficult to conceive as you get older (and can be riskier), you may want to consider how your own age factors in as a decision factor around child spacing.
With the research continuing to evolve on the risks associated with different interpregnancy intervals, it is recommended that you speak with your doctor about your specific circumstances as they relate to planning for child spacing.
Considerations When Adopting
While some states have laws about minimum age differences between you as the adoptive parent and the child you are adopting, there are no state laws in the US that dictate whether an adult is too old to adopt.  The Adoption Network Law Center does note that with private or independent adoptions, birth parents may exercise an age preference for adoptive parents for domestic adoptions within the US. 
For international adoptions, the rules will vary by country. For example, China requires married prospective parents to be between 30 and 55 years old (45 if a single woman adopting). Furthermore, according to the US Department of State, a family “must have not have more than two children in her household under the age of 18 and the youngest child must be at least six years old,” resulting in at least a six-year age gap between the youngest and a new addition if the new addition is an infant. 
With adoptions taking anywhere from a couple of months to a couple of years to complete, plans for spacing out children may need to be flexible—just the same as with trying to conceive. However, given maximum parental ages are not regulated by state laws for adoption, that would allow for more time to wait to add children to your family if you want to extend the time in between sibling additions.
Considerations When Undergoing Fertility Treatments
If you’re planning to grow your family through fertility treatments or think there may be a chance of pursuing that route, you should check in advance with your preferred fertility treatment center about their age requirements. The Advanced Fertility Center of Chicago notes that most fertility clinics have upper age limits on IVF treatment for women using their own eggs (with the average upper limit between 42 and 45 years-old), as well as upper age limits on IVF treatment regardless of egg donor age (some having an upper bound of 50 or older, but others have earlier limits).  With this in mind, you may want to consider how your own age factors into the child spacing question regardless of whether you’re using fertility treatments or not.
Benefits of Different Age Gaps
While this is not an exhaustive list, here’s a roundup of some benefits for siblings that are spaced with more or less time in between sibling additions.
Benefits of having siblings close together in age:
- Limiting time as an only child – The closer together in age siblings are, the less time the oldest has as an only child without a close-in-age playmate at home and the less time the youngest has alone with the parents once their older sibling(s) leave the nest
- Common interests – Children closer in age to each other may have more shared interests, plus closer developmental timelines mean easier ability to play together
- Shared experiences – Especially in big life transitions (e.g., becoming parents themselves), siblings close in age may be more likely to go through transitions over similar times which may enhance their connection 
- Career planning for extended time off – If you or your partner are planning to take extended time off (say, until the kids are in school), then shorter birth intervals consolidate that time out of the workforce
- Keep the gear – With kids close together in age, you won’t be tempted to ditch all the baby gear in between (or let it get out of date!)
- Stay in the groove – Once you’ve perfected the art of diaper changing, why lose the magic touch with a long interval before the next baby?
Benefits of spreading siblings out in age:
- Increased time as an only child – The further apart in age the siblings are, the more time the oldest has as an only child to develop a special bond with parents
- More individualized time for the youngest – Observational studies have found that for families with four or more years between infants, mothers are more likely to devote as high a level of attention to the new infant as they did the first 
- Financial planning – More time in between children gives you more time in between big financial outlays, such as supporting college tuition payments
- Delaying the empty nest – If you want to delay being an empty nester as long as possible, more time in between siblings means more time with the youngest at home before they fly the nest
- One baby at a time – With a longer interval, the odds of the older sibling(s) sleeping through the night and being more self-sufficient by the time baby comes around are also higher
- Less competition – With different developmental timetables, there’s less opportunity for siblings to directly compare their abilities to each other and less competition for parental attention (the latter peaking for children born about two years apart) 
- Career advancement in between children – With more time in children, you will have more time to advance your career should you wish before taking subsequent parental leave
While my fiancé and I both have about two-year gaps with our own siblings, I look at my parents’ siblings and am reassured of the benefits of both shorter or longer intervals between kids. My parents are both middle children of three siblings but with very different spacing. My dad’s mom had all three boys in under five years; my mom’s mom had over six years between my uncle and my mom, then less than two between my mom and my aunt. Sure, my dad’s descriptions of having two playmates essentially his age growing up sound different than my mom’s accounts of having one her age and one a few years older, but they both have strong relationships with their siblings in their own unique ways.
No matter how or when you decide to grow your family, have some patience if the timing doesn’t work out exactly how you imagined. And enjoy the many benefits of whatever spacing comes your way! Even if that means your baby is due during a heat wave. 😊
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