Special patient groups in clinical trials explained

What does "special population" mean in a clinical trial?

A special population is a group of trial participants who need extra protection or a different study design because of physiological, developmental or legal differences from typical adult participants. This includes pregnant and breastfeeding women, children, older adults, and people with reduced kidney or liver function^[1]. These groups are defined this way because a medicine's safety and efficacy profile can differ substantially from the general adult population studied in most early-phase trials^[2].

The underlying concern is evidence, not just ethics. If a group is excluded from trials, doctors later have to prescribe medicines to that group without solid data on the right dose, the right monitoring, or the right precautions^[1]. This is why regulators and research bodies now actively push for the inclusion of under-served groups – including ethnic minorities and socioeconomically disadvantaged groups alongside pregnant women, children and older people – rather than automatic exclusion^[3]. The regulatory definition itself serves a practical purpose. Naming a group as "special" formally raises awareness of its vulnerabilities. It also creates the legal basis for the extra protective requirements that follow^[2].

How are pregnant women included in clinical trials?

As a general rule, pregnant women are excluded from a clinical trial unless the medicine being studied is actually intended for use during pregnancy. Where a trial is designed for use in pregnancy, reproductive toxicity studies are carried out first. Special attention is also given to the study design before any pregnant woman is enrolled^[1].

An analysis of the International Clinical Trials Registry Platform (ICTRP) found that only 4% of clinical trials conducted over the past decade allowed the inclusion of pregnant women. As a direct result, many pregnant women are left without tested treatment options. They often end up taking prescription medicines off-label, without data confirming they are safe to use in pregnancy^[3].

Pregnancy introduces physiological changes that can affect how a medicine is absorbed, distributed, metabolised and eliminated by the body. This is why a dose that works for a non-pregnant adult may not be correct during pregnancy. The World Health Organization has set up a global Task Force with the goal of achieving timely and ethical inclusion of pregnant and breastfeeding women in clinical research by 2030. This effort builds on ongoing disease-specific work as well as the World Health Assembly resolution WHA75.8 and the vision of the WHO Global Clinical Trials Forum, which explicitly calls for the inclusion of underrepresented groups, particularly pregnant women, children and older people, in clinical research. The Task Force's remit reflects a broader shift in thinking, with WHO describing a need for the paradigm around including pregnant women in research to change^[3].

What happens if you become pregnant while already taking part in a trial?

If a woman becomes pregnant while receiving a study medicine that is not intended for use in pregnancy, treatment should generally be stopped, provided this can be done safely. If you find out you are pregnant during a trial, tell the research team immediately so they can assess whether stopping the medicine safely is possible and what follow-up is needed^[1].

What monitoring applies to breastfeeding women and their babies?

When breastfeeding mothers take part in a clinical trial, their children should be monitored for effects of the medicine, because the medicine or its breakdown products (metabolites) can pass into breast milk. If you are breastfeeding and considering a trial, ask the research team specifically what is known about excretion of the study medicine into breast milk and what monitoring your baby will receive^[1].

Why does the exclusion of pregnant women from trials matter for maternal health?

The pipeline for new maternal health treatments is described as sparse and slow-moving. The median time for a new product to reach even 20% uptake in low- and middle-income countries is 13.5 years. A woman currently dies every two minutes from causes related to pregnancy or childbirth, which is part of why WHO describes closing this evidence gap as urgent rather than optional. Slow, sparse pipelines and near-total exclusion from trials compound each other. Without inclusion, there is little pregnancy-specific data to support new products. And without a stronger pipeline, there is little incentive to invest in inclusion in the first place^[3].

How are children defined and grouped in pediatric clinical trials?

Children are recognised as a vulnerable population in clinical research because they differ from adults in development, physiology and psychology. Trials involving them are necessary to improve the treatments available specifically to them. Pediatric age groups are defined in completed days, months or years, as shown in the table below^[1].

Age category	Definition
Preterm newborn infants	Born before term (age counted from birth, adjusted for prematurity)
Term newborn infants	0 to 27 days
Infants and toddlers	28 days to 23 months
Children	2 to 11 years
Adolescents	12 to 16/18 years, depending on region

Source: EUPATI Toolbox, Special populations in clinical studies^[1].

Who can consent on behalf of a child under 16 in a clinical trial of a medicine?

In the UK, the Medicines for Human Use (Clinical Trials) Regulations prohibit children under 16 from giving their own consent to take part in a Clinical Trial of an Investigational Medicinal Product (CTIMP). Instead, consent can be given on the child's behalf by one of the following people^[4].

• A parent or someone with parental responsibility – agreement of only one parent is required^[4];
• A personal legal representative – someone not connected with the trial who is suitable to act by virtue of their relationship with the child, approached only if a parent cannot be contacted in time because of the urgent nature of the treatment^[4];
• A professional legal representative – a doctor responsible for the child's medical treatment who is independent of the study, or a person nominated by the healthcare provider, used only if no personal legal representative is available^[4].

Whoever gives consent must be told clearly that they are consenting on the child's behalf. They must understand the objectives, risks and inconveniences of the trial and how it will be conducted. They must be informed of the right to withdraw the child at any time. And they must have a contact point for further information about the trial. As a parent or carer, you should expect to receive all of this information in writing before you are asked to sign anything. This tiered hierarchy – parent first, personal legal representative second, professional legal representative last – exists specifically to make sure that consent is sought from someone close to the child whenever time allows. It only falls back on a clinician when genuine urgency makes that impossible^[4].

Should the child be involved in the decision to take part?

Children and young people should be involved in the decision-making process whenever possible, and they should receive information about the trial in a form that is understandable to them. As a parent, ask the research team for an age-appropriate information sheet for your child, separate from the document written for you^[4].

Can a young person over 16 consent for themselves?

Young people over 16 are presumed capable of giving their own consent to take part in a CTIMP. If a young person over 16 is not capable of giving consent, they should only be included in a CTIMP under the same provisions that apply to adults who lack capacity^[4].

Does the same consent rule apply to all research involving children?

There is no statute in England, Wales or Northern Ireland that specifically governs a child's right to consent to research other than a Clinical Trial of an Investigational Medicinal Product. This means consent rules for non-CTIMP research (for example, research that does not test a medicine) differ from the rules described above. Separately, common law presumes that young people aged 16 to 18 are usually competent to consent to treatment. A younger child may also be considered competent to consent to treatment if they have sufficient understanding to fully grasp what is proposed and can weigh the information to reach a decision – this is known as being "Gillick competent." If you are unsure which consent framework applies to a specific study your child has been invited to join, ask the research team to explain in writing whether the study is a CTIMP. This gap between the statutory CTIMP framework and the common-law Gillick test is worth noting carefully. The same child, at the same age, could in principle face two different consent pathways. Which pathway applies depends purely on whether the study involves an investigational medicine^[4].

Why do older adults need special consideration in clinical trials?

Older people require special consideration in clinical studies because of the frequent occurrence of underlying diseases, the frequent use of several other medicines at the same time, and the resulting risk of interactions between medicines. Older adults are also more likely to have reduced kidney or liver function, which can affect how a medicine is removed from the body – a separate special-population consideration in its own right^[1].

The WHO Global Clinical Trials Forum explicitly names older persons, alongside pregnant women and children, as an underrepresented group that clinical research should actively include^[3]. A scoping review of methodological research from the UK and Ireland (published between 2000 and 2021) specifically examined interventions to improve representation of older people, alongside ethnic minority groups, socioeconomically disadvantaged groups, and people with impaired capacity to consent, in randomised controlled trials^[5]. Taken together, multimorbidity, polypharmacy, and organ function changes mean that a single older participant can face several overlapping special-population issues at once. This is one reason trial protocols for this group often need more detailed monitoring plans than those for younger adults^[1].

What legal protections exist for special populations taking part in research?

Special populations – including women (pregnant and non-pregnant), pediatric patients, and older adults – are subject to specific regulatory laws designed to protect them, and these protections must be understood and adhered to when conducting clinical research. Regulation defines "special population" precisely in order to raise awareness of these groups' vulnerabilities and to allow protective regulatory requirements to be created around them^[2].

In Spain, Royal Decree 1090/2015 regulates clinical trials with medicinal products, the ethics committees that review them, and the Spanish Clinical Studies Registry. It explicitly states the need to promote clinical research into orphan medicinal products and medicines aimed at population groups such as children, women and older people, who have traditionally been poorly represented in clinical research. This regulation requires that clinical research be conducted in an environment guaranteeing the protection of participants, in line with the Declaration of Helsinki and the Council of Europe Convention on Human Rights and Biomedicine (the Oviedo Convention)^[6].

For comparison, in the United States, special populations for research purposes are clearly defined within the Code of Federal Regulations, which sets out equivalent protective requirements under a different legal framework. Whether the framework is the UK's Medicines for Human Use Regulations, Spain's Royal Decree 1090/2015, or the US Code of Federal Regulations, the common thread is the same. Each jurisdiction singles out pregnant women, children and older people by name for extra procedural safeguards, rather than leaving their protection to general trial rules^[2].

What can improve participation of under-served groups in trials?

A scoping literature review looked specifically for effective interventions and trial design features that improve representation of under-served groups – ethnic minority groups, socioeconomically disadvantaged groups, older people, and people with impaired capacity to consent – in randomised controlled trials conducted in the UK and Ireland. The review's starting point was the observation that participants in clinical trials often do not reflect the populations who could actually benefit from the treatment being studied. There are known barriers to participation for these groups, but limited evidence on which strategies actually reduce those barriers. That gap between recognised barriers and proven solutions is itself a finding. It shows that naming a group as under-represented is only a first step. Trial teams still need better evidence on which specific design changes – recruitment methods, consent materials, visit scheduling – actually bring more of these participants into studies^[5].

When should you contact the research team or consider withdrawing?

Contact the research team promptly if any of the following applies to you or your child during a trial.

• You become pregnant while receiving a study medicine not intended for use in pregnancy – treatment may need to stop as soon as it can be done safely^[1];
• You are breastfeeding and notice any change in your baby's feeding, behaviour or health during the trial, since the medicine or its metabolites may pass into breast milk^[1];
• As a parent or legal representative, you no longer feel you understand the trial's objectives, risks or conditions – you have the right to have these explained again at any point^[4];
• You wish to withdraw your child from the trial – this right applies at any time, and you should have been given a contact point for this when consent was obtained^[4].

Summary

Pregnant women, children and older adults are treated as special populations in clinical research because their bodies process medicines differently, and because they have historically been excluded from the trials that could have generated the evidence they now need^[1]. For pregnant women, exclusion remains the default outside pregnancy-specific studies, and only 4% of trials over the past decade allowed their inclusion^[3]. For children, consent rules depend on age and on whether the study is a trial of a medicine, with parents, legal representatives, and – from age 16 – young people themselves able to give consent under defined conditions^[4]. For older adults, the main concerns are multiple underlying conditions and interactions between medicines, and representation remains an active area of methodological research^[5].

If you or a family member is invited to take part in a trial as a member of one of these groups, ask directly who can give consent, what happens if circumstances change (such as a pregnancy), what monitoring is planned, and who to contact with concerns. These are standard questions that any research team should be able to answer clearly before you decide^[1].

1. [1] Special populations in clinical studies - EUPATI Toolbox (accessed 2 June 2026) – https://toolbox.eupati.eu/resources/special-populations-in-clinical-studies/
2. [2] https://pmc.ncbi.nlm.nih.gov/articles/PMC4577021/
3. [3] Equity and health: the inclusion of pregnant and breastfeeding women in clinical trials (accessed 2 June 2026) – https://www.who.int/news/item/18-06-2025-equity-and-health--the-inclusion-of-pregnant-and-breastfeeding-women-in-clinical-trials
4. [4] Research involving children - Health Research Authority (accessed 2 June 2026) – https://www.hra.nhs.uk/planning-and-improving-research/policies-standards-legislation/research-involving-children/
5. [5] https://openresearch.nihr.ac.uk/articles/4-12
6. [6] Royal Decree 1090/2015, of 4 December, regulating clinical trials with medicinal products, Ethics Committees for Investigation with medicinal products and the Spanish Clinical Studies Registry (accessed 2 June 2026) – https://www.aemps.gob.es/legislacion/espana/investigacionClinica/docs/Royal-Decree-1090-2015_4-December.pdf