Growing Up Together: Twins in the Growing Up in Ireland ’08 Study at 9-Months Old

Introduction

At nine months, children are in a critical phase of early development. This stage is marked by rapid changes in motor skills, emerging communication, and the beginnings of social interaction. For twins, these milestones are shaped not only by individual temperament and genetics but also by the unique experience of growing up alongside a same-age sibling. The release explores their progress, compares developmental benchmarks to those of their singleton peers, and highlights the interplay between shared and individual growth.

The figures presented are unweighted as some of the twins are not part of the original weighted sample, as such, the results presented should be interpreted with caution and are not nationally representative (see Background Notes for more details). In addition, due to the unequal sample size between the twins and singletons, comparisons are illustrative rather than empirical.

In Cohort '08 Wave 1, data was collected on more than 650 (669) twins or multiples aged nine-months, accounting for 5.9% of the total sample of nine-month-olds.

Being a Twin

As part of the first wave of the survey, a brief section was included to gather information specifically related to twins. This section offered valuable insights that may inform further analysis in subsequent waves.

Mothers reported that 19.4% of their twins were identical and 80.6% were fraternal.

When it came to dressing their twins in matching outfits, 23.9% of mothers said that they dressed each of their twins in matching outfits all of the time, 35.3% said they did it some of the time, while 40.9% of mothers of twins said they never dressed their twins in matching outfits.

Mothers were asked if each baby liked to be with their twin, 82.8% of mothers said that the baby like being with their twin most of the time, and 17.2% said some of the time or hardly ever. They were also asked if the baby did not seem to notice their twin, 38.4% said that the baby did not seem to notice their twin most of the time, with 23.1% saying that this happened some of the time, 38.4% said that this hardly ever or never happened. Mothers were also asked if each of their babies were upset if they were parted from their twin, 16.5% said that the baby was upset if parted from their twin most of the time, 19.4% that the baby was upset if parted from their twin some of the time but most mothers (64%) said that the baby was never or hardly ever upset if parted from their twin.

Pregnancy, Delivery and Breastfeeding

A higher percentage of mothers of twins (21.9%) reported they had undergone medical fertility treatment for their pregnancy compared to parents of singletons (3.2%). Nearly two-thirds (65.7%) of mothers of twins intended getting pregnant at the time they got pregnant compared to 58% of mothers of singleton babies.

Close to three in ten (27.6%) pairs of twins were delivered without medical intervention (e.g. suction assisted birth, forceps assisted birth, caesarean etc.) and unbreeched compared to 60.5% singleton births. More than a third (34%) of mothers of twins had a planned caesarean which was nearly three times more than singleton deliveries (11.8%) and 27.9% had an emergency caesarean which was more than twice the rate of singleton births (12.8%).

Most twins were born between 35 – 39 weeks (75.2%) whereas this accounted for only 36.2% of singleton births. The majority of singleton births happened from week 40 onwards (61.7%). The median length of gestation for a singleton baby was 40 weeks, whereas this was 37 weeks for twin babies (see PxStat table GUIT42 and GUIT28).

More than half of twins (53.9%) were breastfed for some time after birth compared to 60.6% of singleton babies. Of the 351 twins (53.9%) who were ever breastfed, 10.1% (N=35) were still being breastfed at nine months (see PxStat table GUIT35).

Support and Childcare

Mothers were asked if they felt that they got enough help and support from family or friends living outside the household. Two-thirds (66.6%) of mothers of twins said that they got enough support, this was lower than reported by mothers of singleton babies at 71.4%.

A higher percentage (69%) of parents of twins reported that they were minding the given twin themselves rather than being minded by someone else when the baby was nine months old compared to 59.9% of parents of singleton babies.

Mothers who responded that their babies were currently being minded by someone else were then asked the extent to which their choice of childcare was determined by financial constraints. Of these, 42.6% of mothers of twins said that their choice of childcare for the twin baby had been affected by financial constraints compared to 33.5% of mothers of singleton babies.

Mothers were asked where each of their twin babies or singleton baby slept for most of the night. A higher percentage of twin babies slept in a bed with other children (8% compared to 0.9% singleton babies) but fewer twins (5.1%) compared to their singleton peers (11.5%) slept in their parent’s bed for most of the night.

Child Behaviour and Development

A higher percent (10.9%) of mothers of twins reported they had concerns about some aspects of their baby’s behaviour or development compared to mothers of singletons (6.8%).

The infant’s developmental status in the first wave of GUI at nine-months was reported by mothers using the Ages and Stages Questionnaire – ASQ 2 (Squires, Potter & Bricker, 1999; see Background Notes for further details). This questionnaire asks whether the child has yet achieved various milestones under the headings of:

• Communication – showing some understanding of basic commands from parents and making first efforts to engage in communication (e.g. “Does your baby make two similar sounds like ‘ba-ba’, ‘dada’ or ‘ga-ga’?”)
• Gross motor – being able to stand up, sit up and move around with support (e.g. “If you hold both hands just to balance her, does your baby support her own weight while standing?”)
• Fine motor – able to use fingers and thumbs to manipulate small objects (e.g. “Does your baby pick up a small toy with the tips of her thumb and fingers?”)
• Problem solving – showing curiosity in, and some rudimentary understanding of, how the world around them works (e.g. “Does your baby poke at or try to get a crumb that is inside a clear bottle?”)
• Personal social – being able to perform some basic self-care tasks such as feeding and showing some early evidence of co-operation (e.g. “When you dress him/her, does your baby push his/her arm through a sleeve once his/her arm is started in the hole of the sleeve?”)

Infants are given scores based on how many milestones they have achieved so far using the Ages and Stages 10-month-old Questionnaire (see Background Notes for more information). Infants who do not reach a target score may be at risk of delay in that particular set of skills. Figure 1.8 shows the percentage of singletons and twins in GUI who failed to reach the target score for an infant aged 10 months, separately for each skill (see Background Notes for further information).

Across all developmental areas measured by the Ages and Stages Questionnaire, a higher percentage of twin children failed to meet the expected target scores compared with their singleton peers. The most pronounced disparity was observed in gross motor skills, where 39% of twins did not reach the target score, compared to 14.6% of singletons.

Failure rates between twins and singletons were most similar in the fine motor domain, which also had the lowest rates of failure across both groups. In this area, 14.4% of twins fell short of the 10-month milestone, the singleton rate was 9.7%.

Personal-social skills represented the highest area of concern for singleton infants, with 17.6% failing to meet the expected score. However, nearly a third (32.2%) of twins also struggled in this domain.

Communication was the strongest area for singleton infants, with 6.6% failing to meet the target; still lower than the corresponding rate for twins (15.7%).