Women who take the Pill each day have a key brain region which is smaller, experts have found for the first time.

The hypothalamus is crucial for the normal production of hormones and plays a role in mood, appetite, sex drive and sleep.

Researchers at Montefiore Medical Center in New York City scanned the brains of 50 women to make the conclusion.

They saw a 'dramatic difference' in the size of the hypothalamus in women who took birth control pills.

But they said cognitive performance or behaviour did not change - despite the brain region being around six per cent smaller.

However, other preliminary evidence from the same team found a link between the smaller size and anger and depressive symptoms.

The research will be presented at the Radiological Society of North America's annual meeting. It is not yet published in an academic journal.

Women who take the Pill every day have a smaller brain region called the hypothalamus, involved in hormone regulation, scientists find for the first time

MRI of a woman's brain with the hypothalamus in red. Differences in hypothalamus sizes are not visible to the eye when looking at brain scans and take further investigation

Lead author Dr Michael Lipton, said: 'There is a lack of research on the effects of oral contraceptives on this small but essential part of the living human brain.'

He added that his team can confirm, 'for the first time', using the Pill is linked with a 'smaller hypothalamic volume'.

The study recruited 50 healthy women, including 21 taking oral contraceptives and had never had brain injury or mental illness. All underwent brain MRIs.

The hypothalamus in women on birth control were about six per cent smaller than those of other women, Discover reports.

'We found a dramatic difference in the size of the brain structures between women who were taking oral contraceptives and those who were not,' Dr Lipton said.

He told Discover: 'I like to tell people that for all parts of the body, size matters most in the brain.' But he added: 'It may not represent a risk at all.'

Other findings from the study, which Dr Lipton described as 'preliminary', were that smaller hypothalamic volume was linked with greater anger.

There was also a strong correlation between the size of hypothalamic volume and depressive symptoms.

But Dr Lipton found no significant correlation between hypothalamic volume and cognitive performance. It is not clear how this was studied.

The hypothalamus is located at the base of the brain above the pituitary gland and helps regulate bodily functions which are essential for survival.

Disease of the hypothalamus, most often caused by brain injury, surgery, tumours or genetic disorders, can cause insomnia, dehydration and infertility.

Effects of sex hormones, including those in oral contraceptive pills, on the human hypothalamus have never been reported.

Dr Lipton said: 'Oral contraceptive pills (OCPs) do entail real risks that are routinely determined by millions of women to be outweighed by the benefits of OCP use.

'Any potential risk related to our finding is at present not known. It will take more research to reproduce and understand the clinical implications of the effects, if any.'

Academics who were not involved in the research said that the findings should not concern any women on the contraceptive - especially as they are preliminary.

Stephen Evans, professor of pharmacoepidemiology, London School of Hygiene and Tropical Medicine, said: 'Small studies which find “dramatic differences” should be treated extremely cautiously – it is likely that the play of chance in the findings has been underestimated.'

Dr Ali Abbara, clinical senior lecturer in reproductive endocrinology, Imperial College London, said: 'The birth control pill works in part by reducing the normal secretion of reproductive hormones from the hypothalamus, thus this finding is not surprising and would likely spontaneously reverse following cessation of the Pill.'

Derek Hill, professor of medical imaging, University College London, said: 'Even if the difference in volume is real, it doesn’t mean that the contraceptive pill has damaged the brain.

'If a drug does make a part of the brain smaller that could be due to a change in fluid in that region rather than any damage to the brain cells themselves.'

Oral contraceptives are among the most popular forms of birth control and are taken by millions of women in the UK and US.

Around 47million women aged 15-49 in the US reported use of contraceptives from 2015-2017. Of those, 12.6 per cent used the Pill.

While many millions of women take the Pill without any problems, depression is listed as a side-effect in the warnings leaflet included in every packet, along with symptoms such as blood clotting, weight gain, pain and reduced libido.

Most brands of the Pill contain a mix of two synthetic hormones: oestrogen and progestogen. The combined pill gives a dose of both of these; the mini-pill has only progestogen.

Oestrogen prevents pregnancy by stopping ovulation. Progesterone thickens the mucus at the entrance of the womb so that sperm can’t get through to fertilise eggs as well as thinning the lining of the uterus, making it difficult for a fertilised egg to implant itself.

Some studies have begun to look at how the introduction of these synthetic hormones alter brain structures.