The luteal phases follows ovulation, and is relatively constant in all women, with a duration of 10 to 14 days. A temporary collection of cells called the corpus luteum forms on your ovary from the remnants of the follicle and secretes progesterone and a small amount of oestrogen. These hormones cause the uterine lining to thicken in anticipation of a fertilised egg. If pregnancy does not occur, the corpus luteum degenerates, leading to a drop in progesterone levels and the onset of menstruation.

• Hormonal changes: High levels of progesterone and a small amount of oestrogen. If fertilisation has not occurred, both hormone levels drop sharply towards the end of this phase.
• Common symptoms: Premenstrual syndrome (PMS) symptoms such as mood swings, irritability, bloating, breast tenderness, and fatigue. Some may also experience acne flare-ups, changes in appetite, and difficulty sleeping.

How do these changes impact your brain?

Studies have shown that fluctuating hormone levels during the luteal phase cause imbalances in brain activity and reduce the responsiveness of the prefrontal cortex, which is crucial for emotional regulation (Gao et al., 2021; Baehr et al., 2004). Neuroimaging studies have also found that high levels of progesterone during the luteal phase transiently alter communication within and between the brain's intrinsic networks. This results in heightened stress reactivity and a stronger memory for negative experiences. Brain imaging research indicates that two key networks involved in emotion and memory processing—the salience network and the default mode network—become better connected when ovarian hormones peak during this phase. This means that during the mid-luteal phase, women may be more vulnerable to negative events, which are experienced more intensely and are easier to encode and retrieve (Andreano et al., 2018).

At the neurotransmitter level, when progesterone levels increase during the luteal phase, it leads to a rise in allopregnanolone (often referred to as "allo"), a metabolite of progesterone. Allo acts as a potent modulator of the GABA-A receptors in the brain, which are responsible for calming neuronal activity. While this can have a soothing effect at moderate levels, promoting relaxation and reducing anxiety, excessively high levels of allo can paradoxically lead to mood disturbances. This is because, in some individuals, high allo levels may cause the GABA-A receptors to become less responsive over time, leading to a kind of withdrawal effect that can result in heightened anxiety, irritability, and even depressive symptoms. This dysregulation of the GABA system during the luteal phase is one of the reasons why many women experience significant mood fluctuations, ranging from mild anxiety to severe premenstrual dysphoric disorder (PMDD).