This is the first of a three part series about speech and language development in twins. Myself and Ute Limacher-Riebold of Ute’s Intenational Lounge are putting our heads together to look at the research and real-life experiences. Be sure to sign up for email updates so you’ll find out what we’re planning for you!
Rates of twin births have increased (almost doubled) across Europe, Australia and the US over the past forty years. In Ireland in 2016, the rate of twin births had increased by 22% over the previous decade. In that year, 2363 sets of twins were born in Ireland and 79 multiple births. And there has been a long-running thread in the research of an increased risk of mild early language delay in twins. Late language emergence is reported to be much more prevalent in twins (38%) than singletons (20%). Late language emergence is when children’s language development is found to be below what you would expect for their age and in the absence of other issues such as hearing impairment, for example. And in Study #4 (listed at the end), there was a higher proportion of late language emergence in identical (monozygotic) twins (48.1%) than non-identical (dizygotic twins) (32.6%).
Are twins really slower to talk? I’ve had a look at the most recent research I could find to see what it says. (Warning- there’s not a lot of recent, high quality research out there but I’ve done my best.) This post is going to focus on monolingual twins. In the next post, I’ll cover language development in multilingual twins. And in the last post in the series, I’m going to look at the research on the idea of twins having a ‘secret language’.
So while researchers have been reporting an increased risk of mild early language delay in twins for many years, a recent paper involving my colleague Ciara O’Toole points out that not a lot of research has looked at the potential benefits for speech and language development of having a partner in crime along for the ride! So the research itself has been biased. And not only that, but there hasn’t been a lot of recent research in general looking at language development in twins. From birth to 2 years of age, the research shows lower levels of language abilities in twins compared with non-twin children. Research shows similar findings for 3 year old twins. This twinning effect, as it’s called, seems to be more likely to occur in identical than in non-identical twins. And twin boys are more likely than twin girls to show late language emergence.
The main recent high quality study that has looked at speech and language development in twins over a period of time looked at a range of speech and language measures at ages 4 and 6 years. 1255 children from 627 pairs and one twin without a co-twin in Western Australia were involved in the study (Study #1 in the list below). The researchers wanted to find out if this twinning effect at 2 years persisted at 4 and 6 years of age. Essentially the researchers concluded that any twinning effect on speech and language development in twins decreases between the ages of 4 & 6. Except for when it comes to speech development at 6 years of age. The speech of the twins at age 6, was not progressing as fast as their non-twin peers. (It’s more complex than that though. It’s important to remember that a few speech errors on the speech tests that the researchers used at age 6 could be enough to significantly reduce the children’s score. And they may well grow out of those errors that were found to be present at age 6). More good news when it comes to differences between identical and non-identical twins. The disadvantage experienced by identical twins when it comes to speech and language development was largely erased by age 6.
Next up is a study from Ireland (Study # 2). It looks at expressive language development in 185 twins compared with non-twin children at ages 3 and 5. The researchers looked at expressive vocabulary. That’s the number of words children were saying at the two ages. The good news is that although the expressive vocabulary of twins at age 3 was slightly lower than the non-twin children, by age 5 that difference had disappeared.
But, twins born after shorter gestations or at lower birth weights seem to be at an increased risk of having less-developed expressive vocabulary skills at 3 years. Another study (Study # 3 in the list) found that an increased risk of gestational diabetes, taking longer to breathe spontaneously after birth, and growth restriction during twin pregnancies contribute to an increased risk of expressive language delays among twins at 2 years.
What happened to help the children in the Irish study catch up between 3 & 5? Attendance at pre-school with 98% of the children in the study attending a free pre-school year between age 3 & 5. (The other research looking at language development in twins and showing catch-up later between ages 9 & 11 is from before attendance at pre-school was common). Bottom line here? If you’re concerned about your twins’ language development at age 2 or 3, there’s a good chance it’s a transient issue and they will catch up. But if you’re concerned, it makes sense to consult with a speech and language therapist at that stage. The SLT can show you language building strategies that benefit your children at this point. Then, it is a good idea to have a follow-up check by a speech and language therapist at age 5.
Why is there a twinning effect?
Once again, there’s a lack of research. Possible suspects are things like complications before or around the time of birth, and differences in how parents of twins interact with the twins (not expanding on what the children say or doing less shared book reading, for example. But the results of study #4 don’t support the idea of reduced input from mothers as a factor in late language emergence). Other factors include a history of gestational diabetes, taking longer to breathe on their own at birth, growth restriction in utero, being an identical twin, and having one or more siblings outside the twin pair. These have all been identified as risk factors for late language emergence among twins. You can read here & here about language development in children who were born prematurely and/or who had a very low birth weight. (Those posts are not about twins but about the impact of low birth weight and prematurity on language development). A lot more research needs to be done to explore the reasons for twinning effects on language development further.
Study # 3 Risks For Late Language Emergence in Twins
Study #3 looked at the before birth (pre-natal) and around the time of birth (peri-natal) risks associated with late language emergence in 473 twin pairs in Western Australia. They report that twins’ early mental and motor development, at 6, 12 and 18 months, has been reported to lag behind non-twin children and to be associated with low birth weight, not family socioeconomic circumstances. But, research findings vary depending on what questions the researchers ask and how they go about answering them. And results aren’t always clear-cut. For example, there are studies involving twins who were premature or had low birth weight and were found to have lower cognitive abilities than non-twin children. And there are studies that took low birth weight and/or prematurity into account when scoring test measures and then found negligible associations between prenatal & perinatal risk factors and late language emergence.
Basically, the results in this study suggest that difficulties in the prenatal & perinatal period are important in causing late language emergence in twins at age 2. They also seek to reassure parents that language issues in twins are not caused by reduced input from parents being busy caring for two babies simultaneously.
What can you do to help?
Caroline Bowen, an Australian speech & language therapist has a list of suggestions for how to help your twins’ language development along. You can find them at the end of her blog post here. The main advice there and from the NHS in the UK is to create opportunities to interact one-to-one with each of your twins as often as you can manage it. (The NHS tips are here.) You don’t have to do anything different for twins other than make sure you get into the habit of addressing them as individuals and not as a unit. You can find a wealth of free language development tips at Talk Nua. To get you started, click here and here for posts on how to build your toddler’s vocabulary
How does your experience fit with the research? Be sure and leave a comment below. And if you haven’t already, please sign up so you can get the next posts about twins delivered straight to your inbox.
Let’s get talking!
What I read so you don’t have to
Study # 1
Rice, M. L., Zubrick, S. R., Taylor, C. L., Hoffman, L., & Gayán, J. (2018). Longitudinal study of language and speech of twins at 4 and 6 years: Twinning effects decrease, zygosity effects disappear, and heritability increases. Journal of Speech, Language, and Hearing Research, 61(1), 1–15. https://doi.org/10.1044/ 2017_JSLHR-L-16-0366
Study # 2
Culloty, A. O Toole, C. & Gibbon, F. (2019). Longitudinal Study of Expressive Language and Speech of Twins at 3 and 5 Years: Outgrowing a Twinning Effect. Journal of Speech, Language, & Hearing Research 62: 2425-2437.
Study # 3
Taylor, C., Rice, M., Christensen, D., Blair, E., & Zubrick, S. (2018). Prenatal and perinatal risks for late language emergence in a population-level sample of twins at age 2. BMC Pediatrics 18 (41): 1-9.
Study # 4
Rice, M. L., Zubrick, S. R., Taylor, C. L., Gayán, J., & Bontempo, D. E. (2014). Late language emergence in 24-month-old twins: Heritable and increased risk for late language emergence in twins. Journal of Speech, Language, and Hearing Research, 57(3), 917–928.
Irish Multiple Births Association http://www.imba.ie/multiples-info/pregnancy-birth/statistics/