Latest findings from PsychGen

 Birth week and birth size show different links to child development  

In a large study of more than 92,000 term-born children, researchers found that being born at different weeks within the 37–41 week range was linked with differences in early-childhood language and motor skills. However, these differences became much smaller as the children grew older and were minimal by around five years of age.

A different pattern was seen for babies who were smaller at birth than expected for how far along in the pregnancy they were. These children tended to have lower scores in some school subjects, particularly mathematics and reading, compared to children that were larger at birth, and the differences could still be seen through adolescence.

This study, published in AJOG, was led by researchers from the University of Warwick and the Norwegian Institute of Public Health using information from the Norwegian Mother, Father and Child Cohort Study (MoBa).

Being born earlier was mainly linked with differences in early development

Children born at 37 or 38 weeks tended to have slightly lower language and motor skills in their early years than children born later within the term period.

However, these differences became much smaller as the children grew older. By around the age of five, there was little difference between the groups.

The researchers also looked at school performance during adolescence. They found little evidence that being born a few weeks earlier or later within the term period was linked with how well children performed at school.

Being smaller at birth was linked with differences that lasted into adolescence

The researchers also looked at how big babies were at birth. Some children were smaller than expected for their stage of pregnancy, meaning that they weighed less than most babies of the same sex who were born at the same stage of pregnancy.

During adolescence, these children tended to have lower scores in some school subjects compared to children born within the expected size range, particularly in mathematics and reading. For example, children who were smaller than expected at birth had lower average mathematics scores at around 10–11, 13–14 and 14–15 years of age.

Children who were larger than expected for their stage of pregnancy did not have poorer developmental outcomes or school performance in this study. There was some evidence of slightly higher mathematics scores at around 14–15 years of age.

“What surprised us was that, while the differences linked with the week of birth became much smaller with age, some differences linked with being smaller at birth could still be seen during adolescence,” says Xuan Zhao, a researcher from the University of Warwick.

What does this mean for children and families?

The results suggest that when a baby is born and how big they are at birth may be linked with children's development in different ways.

Being born a few weeks earlier within the normal term period was mainly linked with differences in development during the early years; these differences became much smaller as children grew older. In contrast, being smaller than expected for their stage of pregnancy was linked with longer-lasting differences in academic performance into the teenage years.

The findings do not mean that babies should be born later or earlier based on these results. Decisions about when a baby should be born at term need to consider the health of both the mother and baby, as well as other medical factors.

“These findings show that when a baby is born and how big they are at birth are linked with children's development in different ways. Following children from infancy into adolescence allowed us to see how these patterns changed as children grew,” says Dieter Wolke, Professor of Developmental Psychology and Individual Differences at the University of Warwick.

QUENBY, M. S., BRANDLISTUEN, P. R. E., HAVDAHL, P. A. K., & ASK, P. H. (2026). The relationship between gestational age, delivery, and fetal size, and neurodevelopment outcomes from infancy to adolescence. American Journal of Obstetrics and Gynecology.

Understanding the Long-Term Psychological Effects of COVID-19 Quarantine

A new study published in BMC Medicine by researchers from Norway and Iceland has investigated the long-term psychological effects of COVID-19 quarantine on mental health. The research, led by Li Lu from the University of Oslo and PsychGen, analyzed data from 105,344 participants across three longitudinal cohorts in Norway and Iceland.

The study found that individuals who had been quarantined during the pandemic showed higher prevalence rates of depression and anxiety, with these conditions defined as scoring 10 or more on the PHQ-9 and GAD-7 screening tools respectively. Notably, the duration of quarantine was associated with increased mental health symptoms in a dose-response manner. Those quarantined for more than 4 weeks had a 72% higher prevalence of depression and a 76% higher prevalence of anxiety compared to those who had not been quarantined.

The longitudinal analysis, with an average follow-up period of 20.5 months, confirmed that these effects persisted over time. Interestingly, the association between quarantine and depression was stronger among females, individuals aged 35-64 years, and those without a COVID-19 infection.

This research aligns with the PsychGen center's aim to understand how environmental factors, such as quarantine experiences, influence mental health outcomes. By examining how these stressors affect different population groups, the study provides valuable insights for developing targeted interventions to support mental well-being during future public health crises.

Strengths: The study's large sample size across multiple cohorts, longitudinal design, and use of validated assessment tools for measuring mental health symptoms provide robust evidence for the findings.

Limitations: The study relied on self-reported data, which may be subject to recall bias. Additionally, the cohorts had different demographic compositions, and the generalizability to other populations may be limited.

Publication: Lu, L., Wang, Y., Ebrahimi, O.V. et al. Long-term psychological effects of COVID-19-related quarantine: an observational study of three cohorts in Norway and Iceland. BMC Med 23, 519 (2025). https://doi.org/10.1186/s12916-025-04349-8 

New Research Identifies Distinct Types of Postpartum Depression

Researchers have identified nine different subtypes of postpartum depression (PPD) in a large Norwegian study involving 7,859 women. The study was led by Anna E. Bauer at the University of North Carolina in collaboration with researchers from the PsychGen Center. They used advanced statistical methods to analyze symptom patterns, trauma history, pain during pregnancy, and genetic factors.

The findings reveal that women with PPD can be grouped into different categories based on symptom severity, timing of onset, trauma history, and physical health factors. For example, one subtype (28% of participants) showed severe depression symptoms combined with trauma history and had elevated genetic risk for several psychiatric conditions. Another group (10%) had mild symptoms with a protective genetic profile.

This research aligns with the work of the PsychGen center, which aims to understand how genes and environmental factors influence mental health conditions throughout lifespan. The findings demonstrate the importance of personalized approaches to prevention and treatment of postpartum depression.

The study has strengths including large sample size and validation in independent groups. Limitations include reliance on self-reported data and the relatively homogeneous Norwegian population, which may limit generalizability to more diverse groups.

Read the full publication: Bauer, A.E., Jaholkowski, P., Lu, Y. et al. Identification and validation of postpartum depression subtypes: a population-based cohort study. eClinicalMedicine 89, 2025. https://doi.org/10.1016/j.eclinm.2025.103540