What is the luteal phase?
The luteal phase is the part of the menstrual cycle between ovulation and the start of menstruation (1). During this time, the hormone progesterone is produced and the body prepares for a possible pregnancy.
In the ovary, the eggs grow in structures known as follicles. Once the follicle has released the egg (in a process known as ovulation) the remaining cells of the follicle become larger and take on a characteristic yellow colour forming the corpus luteum. As well as giving the luteal phase its name, the corpus luteum is responsible for the production of progesterone. While progesterone is the main hormone produced by the corpus luteum, other hormones such as estrogen also increase (2).
Progesterone levels peak about halfway through the luteal phase. If the ovulated egg is fertilised, the body will begin to produce human chorionic gonadotropin (hCG). The hormone hCG enables the corpus luteum to keep producing progesterone and support the early pregnancy. If no fertilisation occurs, the corpus luteum will start to break down between 9 and 11 days after ovulation forming the corpus albicans (1). This results in a drop in estrogen and progesterone levels triggering menstruation. These changes in hormone levels are often associated with commonly experienced premenstrual symptoms such as mood changes, acne, bloating, breast tenderness and even uterine cramps towards the end of the cycle.
What does progesterone do during the luteal phase?
In the first half of the cycle, known as the follicular phase, estrogen produced by the growing follicles stimulates the endometrium to thicken in preparation for pregnancy. After ovulation, progesterone must prepare the uterus for the possible implantation of a fertilised egg. Rising levels of progesterone stop the endometrium from thickening further and the glands in the endometrium begin to secrete various chemical signals in response to progesterone making it more receptive to the fertilised egg (3). The rise in progesterone additionally causes a slight increase in body temperature via the hypothalamus, which is the part of the brain that regulates our internal body temperature, with some research suggesting that this may help with implantation (4,5).
Throughout the normal menstrual cycle, the uterus undergoes contractions and relaxations at different intensities. These spontaneous rhythms help move the egg after ovulation, transport sperm to the egg for fertilisation as well as triggering menstruation (6). During the luteal phase, progesterone reduces these contractions to help increase the chance of implantation.
If pregnancy doesn’t occur during a cycle, the drop in progesterone causes the blood vessels in the endometrium to constrict. This results in a breakdown of the tissue and the onset of menstruation.
What does the length of the luteal phase mean?
The length of the luteal phase is the time between ovulation and the onset of menstruation. The luteal phase generally lasts around 12 to 14 days, but between 11 to 17 days is common (7,8). Studies have also shown that an occasional shorter luteal phase (<11 days in length) is not uncommon (8, 9). What is important to remember is that in contrast to the follicular phase of your cycle, which can be variable in length, the luteal phase tends to be a consistent length every cycle (10). A longer luteal phase could be a sign of early pregnancy whereas a shorter length, for example, less than 9 days in length, could be a sign of luteal phase deficiency (LPD) (7). We will discuss LPD more in the next section.
What is luteal phase deficiency?
LPD is a condition where the production of progesterone is below normal, or when the endometrium does not respond to the normal stimulation of progesterone (7). As a result, the endometrium is unable to properly prepare for the implantation of the fertilised egg or maintain a pregnancy after implantation has taken place. LPD has been associated with disorders such as PCOS, endometriosis and hypothyroidism and has been linked to difficulties getting pregnant and miscarriage (11). There is still some uncertainty surrounding the cause and diagnosis of LPD. One of the signs can be a luteal phase that is shorter than normal (7, 11). Generally, this is considered as a luteal phase lasting less than 8 days although progesterone levels can be low even if the luteal phase is a normal length (11, 12). Spotting in between periods is also considered a sign of LPD. This is because progesterone is required to maintain the endometrium and a drop in levels can result in early shedding of the endometrial lining.
Tracking your progesterone levels with inne
If you have a short luteal phase, you may not actually realise there’s a problem or suspect any issues with fertility until you’re unable to conceive. Tracking your progesterone levels with inne can help you identify any irregularities in your progesterone profile or the length of your luteal phase. inne cannot be used for any type of diagnosis though so any concerns you may have should always be discussed with your healthcare provider.
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