If you have ever caught a dose of the common cold or a flu in the few days or two weeks before your period, it is not always a coincidence. News Editor Beth Clifford explores how the immune system is linked to the menstrual cycle.

By Beth Clifford | Mar 24 2026

While not directly connected, the menstrual cycle and immune system are linked to one another due to the regular fluctuations in hormones within the female reproductive system. Research surrounding female reproductive health is continuously evolving and developing, and there is now growing evidence to suggest that one’s immune response can strengthen or weaken depending on which phase of the menstrual cycle the person is in.

As the female sex hormones of oestrogen and progesterone fluctuate throughout the menstrual cycle, this in turn affects the immune system’s functions and therefore influences how susceptible a person is to catching a bacterial or viral infection. In particular, the luteal phase is when the immune response is weakest and typically where one may find themselves more likely to become sick. 

During the follicular phase (between the end of menstruation to the beginning of ovulation), oestrogen levels are high and progesterone levels are low. The immune system tends to have a higher number of antibodies and a greater inflammatory response. What this means is that the immune response is much stronger and more resistant to dangerous pathogens. However, for individuals with autoimmune diseases, this period of increased inflammation can mean that the immune responses may be more heightened towards not just infection but also, the body's own cells. Hence, autoimmune conditions are more prevalent in women or those with high levels of oestrogen.

During ovulation (approximately two weeks before menstruation), oestrogen levels drop dramatically and progesterone begins to rise. This is because when ovulating, an egg is released from one of the ovaries and the body prepares for fertilisation to take place. The immune response will then weaken to prevent white blood cells from attacking a fertilised egg or outside sex cells (i.e. sperm) that it believes to be hostile pathogen. This in turn increases the likelihood of catching an infection.

In the luteal phase (the days after ovulation and leading up to menstruation), oestrogen rises slightly while progesterone peaks. With higher amounts of progesterone, the immune defenses remain low and keep the body more open to infection. It is also important to note therefore that progesterone based contraception may suppress the immune system as the progesterone components bind to the receptors on immune cells.

As menstruation occurs (the beginning of the menstrual cycle when the uterine lining sheds), progesterone decreases and oestrogen levels remain low initially. The body's inflammatory and immune response starts to strengthen itself again, and towards the end of menstruation, the levels of oestrogen rise and the cycle starts over.

This information is integral to the progression of the study of female reproductive health, but also so that women and people can understand when and why they are more susceptible to viruses or bacterial infections such as STDs. In deepening the comprehension of how the female reproductive system works, individuals are given greater awareness and knowledge of putting in place and ensuring preventative measures that protect their bodies from pathogens or also knowing when their autoimmune conditions are most likely to flare up.