Fertility phases of the menstrual cycle explained, with information on the luteal and follicular phases and ovulation
A woman's menstrual cycle is affected by a complex interplay of hormones that prepare the body for ovulation. By understanding the different phases of the menstrual cycle you can begin to build up a picture of your own fertility pattern and consequently your time of peak fertility when you are most likely to conceive.
The average length
The first day of your period is considered to be day one of your menstrual cycle and although a 'typical' cycle is considered to be 28 days in length, this varies greatly between women with anything between 25-34 days being considered 'normal'. Cycle length can also vary from month to month, with up to 8 days variation between cycles still considered as a regular menstrual cycle.
The two menstrual phases
There are two distinct phases of the menstrual cycle, the follicular phase and the luteal phase. These are separated by ovulation and so are often known as the pre-ovulation and post-ovulation phases respectively.
The follicular phase
The start of the follicular phase coincides with the first day of the menstrual cycle and encompasses the time of menstrual bleeding. This phase is known as the pre-ovulation phase because it is the hormonal changes that happen during this time that prepare the body for, and eventually lead to ovulation. The length of the follicular phase can vary greatly between cycles as it is dependent on the development of egg bearing follicles in the ovaries - typically it can last anywhere between 13 and 18 days.
During the follicular phase the pituitary gland releases FSH (follicle stimulating hormone) which causes ovarian follicles to develop. Eventually one of these follicles becomes dominant and releases an egg. The maturation of the dominant follicle leading up to ovulation causes an associated increase in the hormone oestrogen. This in turn causes cervical mucus to become more facilitating to the movement of sperm and a thickening and nourishment of the lining of the uterus.
Your fertility increases throughout the follicular phase with the most fertile time considered to span 5 days before and 2 days after ovulation, with the most fertile day being the one before you ovulate. This is because the egg can be fertilised for up to 48 hours after leaving the follicle and sperm is able to survive in your body for between 3 and 7 days after being released.
The end of the follicular phase is marked by ovulation. While this typically occurs mid-cycle, its timing depends on the length of the follicular phase.
Once oestrogen production reaches a certain level, it signals to the pituitary gland that the egg is now ready to be released by the dominant follicle. The pituitary gland then produces the hormone LH (luteinising hormone) which triggers ovulation. The egg is released into the fallopian tube where it is ready to be fertilised by sperm.
The luteal phase
As ovulation marks the end of the follicular phase, it also marks the start of the luteal phase. This phase is known as such because of the key role the corpus luteum plays in this part of the menstrual cycle. The corpus luteum is the now empty dominant follicle that released the egg. After ovulation it releases the hormone progesterone which maintains the receptive uterine environment in case of fertilisation and implantation.
Fertilisation and implantation
If the egg is fertilised and implants in the uterus, the corpus luteum will continue to produce progesterone to nourish the embryo until the placenta begins to develop at the start of the second trimester. However, if the egg is not fertilised and does not implant, the corpus luteum will disintegrate after approximately two weeks, causing the drop in progesterone levels that signals the start of menstruation and the beginning of a new cycle.
Although the length of the luteal phase varies between women and can be anywhere between 12 and 16 days on average, it is this phase of the menstrual cycle that remains constant (i.e. always lasts 15 days). If an egg is not fertilised within 24 hours of ovulation, the rest of the luteal phase is considered to be relatively infertile.
Charting your cycle
Methods of charting the phases of the menstrual cycle can help you to predict when ovulation occurs. These include charting your basal body temperature (BBT - this tells you when ovulation has occurred), cervical mucus and cervical position (these help to predict when ovulation will occur). Additionally, hormone testing kits are also available to help you work out when you are most likely to conceive.
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