Fertilization is the meeting of two cells: female (egg) and male (sperm).

The woman has about 400,000 eggs

A woman has about 400,000 reproductive cells (eggs or oocytes) stored from birth in the ovaries, which are the size of a prune on either side of the uterus. From puberty onwards, an egg is released each month, a total of 300 to 500 eggs before menopause.

Three months to make a sperm

The testicles contain thousands of tubes in which sperm are made, which takes 74 days, or about 3 months. These are stored in two reservoirs, the seminal vesicles, and are then released during ejaculation.


As soon as the cycle begins, a small bulge appears on the surface of the ovary. This is the grape-like follicle that contains the egg. Around the 14th day, most often, under the influence of pituitary hormones, the follicle ruptures, ovulation, occurs. The egg is collected by a tube (a tube that connects the ovary to the uterus) where it will live for a few hours. After ovulation, the follicle turns into a corpus luteum that secretes progesterone, a hormone that helps to establish and maintain pregnancy. This progesterone also has the property of raising the temperature, which is used to detect ovulation.


During ejaculation, several tens of millions of sperm are released into the vagina upon contact with the cervix, which secretes abundant cervical mucus in the middle of the month. A small proportion manage to migrate into the womb by migrating into the mucus. From there some progress in the fallopian tubes in a few tens of minutes. The others are stored in the womb where they will survive for several days, then migrate into the fallopian tubes. A few hundred sperm surround the oocyte in the outer third of the tube and only one of them will enter the oocyte.

Sperm cells such as oocytes are the only cells in the body that have only 23 chromosomes, instead of 46 for all the other cells in our body. The 23 chromosomes contained in the sperm will merge with the 23 chromosomes of the maternal cell to form the baby’s first cell with 46 chromosomes, half of which provide the characteristics of his mother and the other 23 chromosomes the characteristics inherited from his father. This first cell will split into two cells, then four, sixteen… Thus the egg is formed, which is also called embryo; a new being who inherits the characters of his parents is created and will develop over 9 months.

This embryo progresses in 3 or 4 days in the tube, thanks to small eyelashes, up to the uterus. After 2 or 3 days of floating in the uterus, it will nestle in the mucous membrane that covers the inside of the uterus called the endometrium. It’s the nidation. The egg develops extensions that will cling to the uterine mucosa and become the future placenta that will serve as a lung and also as a digestive tract for the fetus.

The special case of twins

It is during fertilization that twins are formed with two categories:

– For the fraternal twins, during ovulation, two eggs were released, each of which was fertilized by a sperm cell. There may be two children of different sexes or of the same sex but only resembling each other as brothers and sisters

– In the case of identical twins, an egg is fertilized by a sperm; then the egg will be divided in two and these two halves will grow separately to produce twins of the same sex that will look very similar.

2 – Nidation

Under the influence of hormones, estrogens and then progesterone, the lining of the uterus will thicken. On the 7th day of life, the embryo adhered and nested in the thickness of the mucosa. At this level, there are more and more vessels and glands secrete nutrients. Small filaments, chorionic villi sink into the lining of the uterus. This is the future placenta. The embryo will grow at a rapid rate and gradually isolate itself. At one month of age, six weeks after the last menstruation, the fetus measures 2 to 4 mm; we can observe in ultrasound the beats of what will be its heart.

3 – Girl or boy

Our chromosomes have 46 pairs grouped into 22 identical pairs in men and women. The twenty-third pair of chromosomes known as the sex chromosome contains the XY chromosomes in men and XX in women. The eggs have only 23 chromosomes, including a sex chromosome, which is always an X chromosome. Sperm also have only 22 chromosomes plus a sex chromosome that will be either type X or type Y. It is the fertile sperm that determines the sex of the unborn child. If the sperm has the X chromosome, the embryo will have 46 pairs of chromosomes, two of which will come from the father and one from the mother, and it will be a daughter. If the fertile sperm is Y, the embryo will have 46 pairs of chromosomes, including one X chromosome from the mother and one Y chromosome from the father. So he will be a boy. It is therefore a game of chance that will cause the sperm to determine the sex of the girl or boy. So there is a 50-50 chance that you will have a boy or a girl. In fact, in reality, there are slightly more boys than girls (1.05 boys for every girl).

Sex selection

Several methods have been proposed to try to choose the sex, either by special diet in the mother, or by modifying the vaginal environment, for example by acidifying it, or according to the date of intercourse in relation to ovulation. In practice, none of these methods have proven their reliability. And maybe it’s for the best!

What changes in the mother

1 – The first signs of pregnancy

(a) Delayed rules

For most women, the first indicator is the delay in menstruation, but it is not always easy to recognize, especially if menstruation is irregular or if you have just stopped taking the pill. In these cases, ovulation, and therefore the beginning of pregnancy, can be delayed.

In addition, it is not uncommon for fairly frequent bleeding during the implantation of the egg in the uterus at the beginning of pregnancy to be mistaken for menstruation.

(b) Other signs

Nausea is frequent but not mandatory. The same applies to other signs: breast tension, weight in the lower abdomen, drowsiness, need to urinate more often, disgust with certain foods or smells, mood swings, constipation, etc. These signs do not guarantee that you are pregnant and their absence does not rule out pregnancy. So be careful.

(c) Temperature rise

During ovulation, the temperature rises by 4/10th of a degree or more above the temperature of the days before ovulation and above 37° C most often. If menstruation occurs, the temperature drops. In case of pregnancy, the temperature remains above 37°C. In practice, this sign is only reliable if you have made the temperature curve every day since the last period. In this case, after the temperature rise, if it remains in a “plateau” for more than 18 days, it is likely that you are pregnant, which is prudent to confirm with a pregnancy test.