Every pregnancy begins with an encounter: that of an oocyte and a sperm. These are the essential ingredients for getting pregnant. From ovulation to implantation, including fertilization: here are the different stages of the conception of the baby.

At birth, every baby girl is born with a stock of several hundred thousand follicles in the ovaries. Each month, around twenty follicles start to grow during the first part of the cycle (follicular phase), under the effect of follicle-stimulating hormone (FSH) secreted by the pituitary gland. But within this cohort, only one follicle (or even two in the case of fraternal twins) matures and reaches ovulation.

Under the influence of FSH, cells in the ovary secrete more and more estrogen. This increase in estrogen levels acts on the hypothalamic-pituitary complex and causes the massive secretion of another hormone: luteinizing hormone (LH). It is this LH surge that will trigger ovulation on day 14 of a 28-day cycle. The follicle ruptures, causing the release of the oocyte. Thanks to the movements of the fallopian tube, the oocyte will then migrate to the upper part of the tube where it will eventually be fertilized by a sperm.

At the same time, the ruptured follicle turns into a corpus luteum. During the second phase of the cycle (lutheal phase), the uterine lining will prepare for possible implantation under the effect of progesterone and estrogen secreted by this corpus luteum.

Unlike the female cycle, spermatogenesis (production of sperm) is a continuous process. It takes place in the testes, from stem cells (spermatogonia) which multiply by cell division. The process takes about 74 days, then 12 days in transit.

Each sperm is made up of three parts:

• the head which contains the nucleus (which itself contains the chromosomes) and a membrane called the acrosome;
• an intermediate part of which certain cells (the mitochondria) will provide energy to the spermatozoon;
• the flagellum which will allow it to move.

 During sexual intercourse, after ejaculation, millions of sperm are found in the vagina. They will then migrate through the cervical mucus, cross the cervix and reach the outer third of each tube after about 2 hours. Of the 60 to 500 million sperm present at the start, only 100 to 200 will arrive at their destination.

Under the effect of an enzyme, the follicular cells attached to the oocyte will gradually detach themselves. Several spermatozoa will then enter the zona pellucida surrounding the oocyte, thanks to enzymes released by their acrosome, but only one will be able to cross the barrier of the oocyte.

This is when fertilization takes place: the membranes of the spermatozoon and the oocyte merge, then the sperm enters the oocyte. The nuclei of the two gametes increase, come together and eventually merge. They then form an egg, called a zygote. It is the first cell of the embryo, which carries all the genetic material necessary to form a human being.

The “window of fertility” is quite short: only 3 to 5 days per month. It depends both on the lifespan of the ovulated oocyte, and on that of the spermatozoa.

• once in the tube, the oocyte is only fertilizable within 12 to 24 hours. Once this period has passed, it spontaneously degenerates;
• sperm can remain fertilizing for 3 to 5 days.

Fertilization can only take place when the oocyte can be fertilized, so up to 12 to 24 hours after ovulation. But it can be fertilized by sperm that have remained fertilizing after intercourse that took place before ovulation. The fertility window, that is to say the period during which intercourse can potentially lead to fertilization, is therefore between 3 to 5 days before ovulation (depending on the length of the sperm’s life) and 12 to 24 hours after ovulation (depending on the lifespan of the oocyte).

To put the odds on your side, it therefore seems a good idea to have at least one intercourse 1 or 2 days before ovulation, then another on the day of ovulation.

Once the egg is fertilized, it must then migrate to the uterus via the tube. This journey takes about 3 days, during which the egg will begin to segment: its cells divide and multiply. Once in the uterine cavity, the egg will float there for 2-3 days.

7 days after fertilization, implantation takes place: the egg enters the uterine lining. This implantation will continue throughout the second week of pregnancy.

Upon implantation, the cell layer of the egg secretes a specific hormone: chorionic gonadotropin hormone (HCG), which keeps the corpus luteum active for 3 months. It is precisely this hormone specific to early pregnancy that pregnancy tests detect in the urine or in the blood.