To better understand the difficulties associated with infertility, you need to comprehend the process of natural reproduction. Of course, everyone knows how babies are made, but are you sure you know the complex physiological and chemical mechanisms behind this moment of pleasure that leads to the birth of a new human being?

Due to the complexity of the mechanism, many anomalies are possible, each of them can lead to temporary or permanent infertility.

Every sexual birth is the result of two gametes (or sex cells) of two different sexes.

Gametes must therefore be produced:

for male, it will be sperm,

for female, it will be eggs.

Sperm production

For males, spermatozoa are produced continuously and permanently in the testicles from puberty onwards. They mix with various secretions from adnexal glands (seminal vesicles, bulbourethral glands and the prostate) to form semen.  Semen contains nutrients (mainly a sugar called fructose, which gives energy to the spermatozoa) as well as chemical substances that activate, protect and help them to move.

Each time a man ejaculates, he expels several dozen to several hundred million spermatozoa in a volume of 2 to 6 millilitres. Sperm quality varies from person to person, and from period to period. It can be strongly affected by the environment (pollution, etc.) but also by certain behaviours or conditions: stress, illness, smoking, thermal conditions, etc.

Egg production

Eggs, or female gametes, are produced by the woman's two ovaries.

From birth, every woman has a definitive stock of eggs, varying from 250,000 to 500,000 per ovary. This "stock", known as the ovarian reserve, diminishes over the years, becoming virtually nil at the menopause. Of this gigantic stock, only a few hundred will be released from ovaries for eventual fertilisation during a woman's reproductive life.

At puberty, the ovaries are activated by various hormones to set up a 28-day cycle that enables fertilisation and nesting.

A regular female cycle lasts on average 28 days and is made up of two phases:

- the follicular phase lasts around 14 days and ends with ovulation. During that phase, ten or so follicles that exist in each cycle on each ovary, known as the dominant follicle, develops in the ovary under the effect of hormones called gonadotropins LH and FSH. Around the 14th day, when it has reached the mature size of around 20 mm, the oocyte, ready to be fertilised, is released. This is ovulation.

This fertilisable oocyte is expelled towards the fallopian tube where it is placed on the mobile fringes.

The luteal phase: this begins after ovulation and ends with menstruations. This phase also lasts around 14 days. The follicle containing the released egg then transforms into a corpus luteum which, if fertilisation has occurred, will release hormones to prepare the uterus for implantation of an embryo. This is the implantation. In the absence of fertilisation, the corpus luteum will destroy itself, triggering menstruations, which is a detachment of the uterine lining accompanied by bleeding. The purpose is to regenerate the lining for the next cycle.

These monthly cycles end at the menopause.

Fertilisation

For fertilisation to occur, the male and female gametes must meet: this is the sexual intercourse, which must take place during the ovulation period. There's no need to describe it to you, you probably know how it's done! To sum up, under the effect of a male orgasm, the spermatozoa are expelled and deposited at the bottom of the vagina. And so, begins the obstacle course for the sperm! Only one will win and have the privilege of fertilising the egg. To accomplish this, it must reach the fertilisable egg in the fallopian tube.

Once dropped in the vagina, the spermatozoa move in with their long tail called the flagellum. They first pass through the cervix thanks to cervical mucus, a viscous, stringy liquid secreted by the cervix only during the period when the woman is fertile. They then enter the uterus and, through contractions of the uterus, travel up to the fallopian tubes where the coveted oocyte is located!

Barely 150,000 spermatozoa reach the fallopian tubes out of several million initially.

The egg is only fertilisable for 24 hours after ovulation and the spermatozoa can remain alive for four to five days once in the vagina. This explains why a woman is only fertile for a few days each month.

Once in the fallopian tubes, a few dozen spermatozoa will stick to the oocyte and only one will pierce its membrane and penetrate the oocyte (the others will not be able to penetrate). The two membranes fuse: the oocyte and the spermatozoon. About twenty hours after fertilisation, the male and female chromosomes are combined to create a new genome in a single cell called a zygote. This is the beginning of the embryo.

Subsequently, this cell divides into 2, 4, 8 etc. then 30 cells on the 3rd or 4th day, 200 cells after 6 or 7 days.

The embryo moves towards the uterus through the contractions of the fallopian tube and about a week after fertilisation, it becomes embedded in the endometrium, the inner lining of the uterus, prepared to house the embryo for 9 months until delivery.

It is easy to understand that any malfunction or disturbance in this complex process can lead to the absence of natural procreation. There are many conditions to be met before conception can take place.

For example, you need to:

- have frequent and regular unprotected sexual intercourse (this is obvious!),

- at least one tube must be functional,

- good quality eggs and sperm,

- spermatozoa that are sufficiently numerous and mobile,

- sexual intercourse taking place during the ovulation period,

- the cervical mucus must be of good quality to allow the spermatozoa to survive,

- the secretion of the hormones FSH and LH by the pituitary gland/hypophysis must not be disrupted,

- the endometrium must be thick enough and of a quality suitable for implantation of the embryo,

- etc.

This list is just a small selection of the disorders that can affect natural conception.

The complexity of natural conception explains why, despite regular sexual activity, pregnancy occurs within a few months to over a year.  For a couple aged between 20 and 25, the fertility rate is around 25%. It falls rapidly after the age of 35, making motherhood more difficult.