The menstrual cycle characterizes female fertility. It can be broken into two phases. The first is the follicular phase. This phase ends with ovulation and is followed by the luteal phase. The luteal phase tends to be fixed in length and makes it possible to predict the next menstrual cycle once ovulation is known. It nominally lasts 14 days.
The Menstrual Cycle
The menstrual cycle is the periodic cycle of female fertility. The beginning is marked by menstruation. At this time the uterine wall, or endometrium, is shed. This will later be built back up to prepare for the possibility of conception during the coming cycle. Each cycle begins with this process and ends with it repeating at the beginning of the next cycle. The menstrual cycle length refers to the number of days between menstruation dates. Although cycle lengths vary between women and from cycle to cycle, the nominal length of 28-days is typically assumed.
The Follicular Phase
During the follicular phase the ovarian follicle is given time to mature. Once maturation completes, an ovum (or egg) is released. This is known as ovulation. In the case of a 28-day cycle with a 14-day luteal phase, this will occur on cycle day 14 (counting from the first day of menses as cycle day 1). Whenever cycle length varies, it is typically due to variations in the follicular phase. This moves out the date of ovulation and makes it difficult to predict without additional information. Some techniques still try to estimate ovulation, but may be ineffective due to this reason.
Ovulation corresponds to the release of an ovum from the ovary. The egg travels down the fallopian tube. It typically survives for approximately 24-hours. Sperm introduced at this time, or still present from previous intercourse even a few days earlier, can fertilize the egg.
More details pertaining to ovulation and its detection are available here.
The Luteal Phase
The time from ovulation to just before the start of the next cycle is the luteal phase. Unlike the follicular phase, this phase is normally fixed in length. The actual length for an individual female can vary, but is around 14-days on average. It tends to be consistent though from cycle to cycle. Variations in cycle length are due to follicular phase fluctuations.
Women charting their fertility sometimes count the luteal phase. The term Days Past Ovulation is often used and abbreviated DPO. The ovulation date is day zero. In the case of a 14-day luteal phase, menstruation should begin on DPO 15. This is because the last day of the luteal phase is the day before the start of the next cycle.
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