Fertility Basics - Reproduction Explained

Natural human reproduction is such an amazing and complex process it seems miraculous that pregnancy occurs as often as it does. The three main phases of human reproduction, as outlined below, are ovulation, fertilization, and implantation.


Each month when a woman begins her menstrual cycle, several eggs begin to grown in both ovaries. Each egg grows inside of a structure called a "follicle."

The body triggers ovulation by releasing a hormone called luteinizing hormone, or LH. This hormone is readily measured in the urine about 12 - 24 hours prior to the release of the egg. Ovulation detection kits can be used to detect the LH surge - the time in a woman's cycle when she is most likely to conceive. During ovulation, the follicle ruptures, releasing the fluid which is contained inside and then releasing the egg.

This egg leaves the ovarian follicle and is "captured" in the end of the fallopian tube. Here, it will begin to move slowly down the tube towards the uterus. However, for a pregnancy to develop it must first meet sperm while it is still held in the fallopian tube. It must be fertilized within the next 24 hours or it will die.

How the sperm gets to the egg

The sperm leave the man's penis by ejaculation and are deposited in the vagina high up near the cervix (the opening to the uterus). The sperm immediately begin swimming, and some will find their way into the cervix.

The sperm then begin their long journey towards the egg. Leaving the cervix they enter the uterus. Here, they swim towards the fallopian tubes. The vagina and the uterus are quite hostile environments for sperm, however, once the sperm reach the fallopian tubes they are mainly free from the potential negative effects of the woman's immune system. Only 1 in 14 million of the ejaculated sperm will reach the fallopian tube, but once there the sperm should pick up chemical signals from the egg to help them find their way forward.

The sperm finally near the egg and push towards its shell (called the zona pellucida). Many sperm will bind to this shell, but only 1 sperm will be allowed to go all the way through to reach the egg inside.

Sperm can survive for two or more days in the cervical mucus and therefore, exact timing of intercourse isn't necessary. A sperm ejaculated during intercourse on a Monday could fertilize an egg ovulated one or two days later.


Once the sperm has gained entry to the egg, a complex chain of events occurs over a period of about 16 hours culminating in the sperm's genetic material forming a structure called the "male pronucleus" and the egg's genetic material forming a "female pronucleus."  The male and female pronuclei move together to the center of the egg and during IVF this can be seen down the microscope. The egg can now be called a fertilized embryo and would normally be at this stage 1 day after ovulation. Embryo development

The following day after fertilization the embryo's genetic material should double then halve forming two identical cells, all still within the shell. This doubling of genetic material and halving continues over the next two days until the embryo is around 8 cells.

At 8 cells the embryo continues to keep dividing to make more cells, but now the cells become very tightly bound and start to communicate with each other.

Around day 4 to 6 after fertilization, the blastocyst stage of the embryo begins. The embryo now has between 50 and 100 cells .Fluid starts to fill inside the embryo making a small cavity. The outer cells begin to form a wall and the inner cells form a ball - this will become the future baby.


All this time the embryo has been moving down the fallopian tube, but at the blastocyst stage the embryo enters the womb. Over the next few days it will hatch out of its shell and begin to bury into the wall of the womb. It will grow and eventually form blood vessel connections with the mother. This stage of connecting with the womb wall is called "implantation" and is another critical stage in achieving a pregnancy.

At this point the embryo is sending out chemicals into the mother's blood stream, and from about 2 weeks from fertilization the chemicals have reached a level which will be picked up by a pregnancy test.

The embryo must then continue to grow and develop the different types of cells and structures necessary to become a baby. The process of conception has occurred and the woman is pregnant.

When to seek help from a fertility doctor

As described above, there are several complicated steps that must occur for a pregnancy to result:

  • the woman ovulate a healthy, mature egg
  • the man produce sufficient healthy, swimming sperm
  • the sperm must reach the egg
  • the egg must change its structure to become a fertilized embryo
  • genetic material of the embryo must be correct
  • the embryo must divide correctly until it forms a blastocyst
  • the blastocyst must implant in the womb without being rejected

It can take time.  But how much time?  According to the American Society of Reproductive Medicine:

Infertility is a disease, defined by the failure to achieve pregnancy after 12 months or more of regular unprotected intercourse. Earlier evaluation and treatment may be justified based on medical history and physical findings and is warranted after 6 months for women over age 35.