How do natural killer cells in the uterus help with conception and pregnancy?

A slight inflammation in the lining of the womb, is necessary for the successful implantation of embryos. The natural killer cells create this inflammation, so they are beneficial. They are also helpful in ensuring that the baby gets the oxygen and nutrients needed in a healthy pregnancy.

How can natural killer cells cause infertility or miscarriage?

There is a belief that too many NK cells can result in miscarriage, due to the higher levels of inflammation and the NK cells attacking the embryo.  

Are there tests for NK levels in the uterus?

Although testing exists, this is not usually available through the NHS and is usually carried out in private clinics. There are also issues with the tests, which you need to be aware of before making a decision to test. The first, comes fromRGOGs scientific paper, 2015, regarding NK levels:

‘Particular interest has been paid to their potential role in pregnancy pathology; specifically the role of uNK cells in recurrent miscarriage (RM) and recurrent implantation failure(RIF). Although several clinical studies have suggested that peripheral blood(PB) natural killer (NK) cells and/or uNK cells are increased in women with RM and RIF, data to date is inconclusive because of significant heterogeneity across studies arising from the use of different methods to quantify NK cells.An understanding of the role of these cells in reproductive failure and their value in clinical practice will not be established until a consensus is reached on how they should be measured.’

There are no official guidelines on what ‘normal’ levels are and the levels differ in each menstrual cycle, so one test may not give a truly representative picture.

What treatments are available for NK cells?

Treatment exists, in the form of suppressing the number and activity of the NK cells in the uterus. However, as too many and too few NK cells have been associated with miscarriage and there is no guideline on what constitutes ‘normal,’ the effectiveness of treatment remains inconclusive.

‘It’s important to know that the Royal College of Obstetricians and Gynaecologists (RCOG) says that there is not enough evidence that these treatments are effective. These medications can also have serious side effects in pregnancy. For example, prednisolone can affect the baby’s growth in the womb.’(Tommy’s, 2020) 

Here at The IVF Network, we understand the challenges faced by individuals and couples struggling with infertility. Through our dedicated channel of experts, our website and our blog posts, we endeavour to provide a wide range of information around infertility, conception and pregnancy, to help you to make informed choices, on your personal fertility journey.

If you are looking for support around miscarriage and baby loss, Tommy’s are a pregnancy charity, funding pioneering research in helping to prevent complications and offer the best care.



‘What are Natural Killer (NK) cells?’  Tommy’s, reviewed 5/6/20


‘Natural Killer Cells and Pregnancy’ First Cry Parenting, Aarohi Achwal, 2020 (medically reviewed by Dr. Sabiha Anjum (Gynaecologist/Obstetrician)


‘The Role of Natural Killer Cells in Human Fertility (Scientific Impact Paper No. 53), RCog (Royal College of Obstetricians and Gynaecologists), November 2015