Menstrual cycle

The menstrual cycle begins on the first day of the menstrual period, the first day when you bleed, and it ends the day before  the next period begins. On average the menstrual cycle lasts 28 days with a normal range between 21 and 35 days. There are two related parts of the menstrual cycle one involving changes in the ovary (ovarian cycle) and the other in the uterus (uterine cycle).

Ovarian cycle

The ovarian cycle is separated into two parts or phases.

• The first part of the ovarian cycle is called the "follicular phase" when the ovary prepares an egg (a follicle) to be released from the ovary. The follicular phase ends with ovulation, the ejection of the egg from the ovary.
• The second part of the ovarian cycle is called the "luteal phase" which begins right after ovulation when there is an increase in progesterone hormone produced by the corpus luteum, the area in the ovary left over after ovulation. In the luteal phase the corpus luteum produces hormones that help the uterine lining to increase in thickness and prepares for a fertilized egg, the embryo, to implant and attaches to the lining. 

Uterine cycle

The uterine cycle is separated into three parts, the menstrual period, the proliferative and the secretory phase.

• The menstrual period is the time when you bleed, when you have a menstrual period if pregnancy and implantation did not occur in the prior cycle.
• The proliferative phase begins with the end of the menstrual period and ends with ovulation.
• The secretory phase begins with ovulation and ends the day before the next menstrual period begins. If you are pregnant, if there is implantation, then the secretory phase does not end with a mentrual period.

Post ovulation

After ovulation, the egg is picked up by the fallopian tube and then moves down the fallopian tube on the way to the uterus. It is inside the fallopian tube, within 12-24 hours after ovulation, that fertilization happens and when the joining of the sperm and the egg occur. 

The corpus luteum is the area in the ovary left over after ovulation and which proiduces the progesterone hormone. The corpus luteum produces the progesterone hormone whose increase  leads to a thickening of the uterine lining, the endometrium, and preparation of the lining for implantation of the embryo. This thick lining is perfect for the implantation of the embryo. The corpus luteum is present in the ovary for up to 14 days after ovulation.

No fertilization and/or implantation

The pregnancy hormone human chorionic gonadotropin (hCG) is being produced when implantation of the embryo happens. If the uterine lining and the corpus luteum are not stimulated by human chorionic gonadotropin, progesterone and estrogen levels fall back to normal.

During the 14 days after ovulation, the body is prepared for egg fertilization and implantation.

When progesterone levels decline if implantation did not occur, then the uterus gets the signal to start shedding that thicker inner layer. This shedding is when you start bleeding, when a normal menstrual period begins. The first day of the menstrual period is when the next menstrual cycle begins. Some women have periods that last just three days and others last seven days or more. Once all the uterine lining is shed, the menstrual period stops and the new fertility cycle begins.

The post ovulation phase is also called the luteal phase and typically lasts between 12 and 14 days. Some women have a short luteal phase lasting less than 12 days. This makes the time frame for conception shorter.

Fertilization and implantation happen

The pregnancy hormone human chorionic gonadotropin (hCG) is being produced when implantation of the embryo happens. If the uterine lining and the corpus luteum are stimulated by human chorionic gonadotropin, then progesterone and estrogen levels continue to rise in support of the pregnancy.

Progesterone is key

If the corpus luteum dies off before the 12-day mark and if fertilization has already occurred, there may not be sufficient progesterone to sustain the pregnancy. In this case, a very early pregnancy loss could occur, but it may appear to be a heavier than normal menstrual cycle if a pregnancy test has not revealed pregnancy.

During the 14 days after ovulation, the body is prepared for egg fertilization and implantation. The uterine lining is thick and estrogen and progesterone levels peak. If fertilization occurs after ovulation, HCG levels will start to increase and the uterine lining will not receive the signal to shed. Most women miss their first menstrual cycle aboutr 2 weeks after ovulation/fertilization, though the pregnancy countdown is typically started from the last day of your last period.