When does fertilization occur? Instantly after having intercourse or not? Does the new life start at that point? Learn all the answers and more!

Fertilization is the beginning of pregnancy. It is a complex process that can take place after sex. However, contrary to what some people think, it does not occur right after sexual intercourse. In fact, fertilization is not instantaneous, but can occur even 3 days after ejaculation.

Fertilization consists of an egg (which is the female gamete) joining with a sperm (male gamete). That is, it is the process by which the genes of both parents combine and a new cell called a zygote is generated.

There are many doubts and myths about when does fertilization occur. Therefore, in this article we explain everything you need to know about fertilization, how long it usually takes until it and when the symptoms of pregnancy begin.

When does fertilization occur?

Fertilization, as we have pointed out in the introduction, is the process by which the ovum and the sperm fuse, giving rise to the zygote. Although it may sound simple, it actually requires a series of optimal conditions to occur.

In fact, as an article by the Southern California Center for Reproductive Medicine explains, the probability of getting pregnant ranges between 20 and 25%. This data refers to a situation in which women do not use a contraceptive method.

This is because, even if fertilization takes place, the gestation process often does not continue. It is important to know that, for it to take place, the woman must be at a specific time of the menstrual cycle.

This is stated by the Assisted Reproduction specialists. This usually happens around day 14 of the menstrual cycle. It is when ovulation occurs. However, sperm can live for up to 3 days in the female genital tract.

The ovum, for its part, only survives 24 hours after ovulation. So, when does fertliziation occur? Hence, fertilization takes place in that period of time (1 day), although the sexual act itself may have occurred two or three days earlier. In the following sections we explain all the phases of this process.

Search for the ovum for fertilization

During the ovulation process, the egg goes from the ovary into the fallopian tubes. Sperm, meanwhile, develop continuously in the testes. When ejaculation occurs, they travel immersed in the semen.

They ascend through the vagina into the fallopian tubes. A process called training takes place there. What happens is that the sperm undergo a series of modifications in their structure and mobility. In this way, they acquire the ability to penetrate the ovum.

Before, just after ejaculation, the sperm are unable to do so. Also, not everyone experiences these transformations. For this reason, the probability that several fertilize the same ovum decreases, since this could lead to an unviable zygote.

Meeting of the ovum and sperm

Once training has occurred, which takes place in crypts of the female genital tract, the sperm is released. It is directed towards the ovum and the so-called acrosomal reaction takes place.

According to an article from the Instituto Bernabeu, this reaction consists of releasing certain enzymes that allow the sperm head to cross the oocyte membrane. In this way, the nuclei of both cells and the genetic material fuse.

During this reaction, a mechanism is also in motion that prevents other sperm from crossing the membrane. As we have pointed out in the previous section, this is essential so that there are no problems at the genetic level and is called polyspermia blocking.

Displacement towards the uterus

Once the egg and sperm fuse, they form a cell called a zygote. It contains genetic material from both parents. Fertilization takes place, in most cases, in one of the fallopian tubes.

That is why the zygote must descend into the uterus, which is the organ in which gestation takes place. During the journey the zygote begins to divide. That is, from that single cell, the embryo is formed from successive cell divisions.

At first, the two cells from the zygote are called blastomeres. On the fourth day after fertilization there are already between 12 and 16 blastomeres. Then the morula is formed, which has a central part that will give rise to the embryo. The outermost layer of cells will form the placenta.

Final part of fertilization: implantation

Implantation is an essential step for the embryo to develop. It is the process by which the embryo attaches itself to the endometrium, which is the lining of the uterus.

It usually occurs between 6 and 10 days after fertilization. If implantation is not successful, the embryo will be expelled with the next period.

When do pregnancy symptoms start?

The first trimester of pregnancy involves a series of symptoms that, in a way, warn the woman that she is pregnant. The first sign is usually a missed or late period, amenorrhea.

Amenorrhea occurs because the production of certain hormones increases to ensure the development of the embryo. These hormones also cause changes in the sensitivity and size of the breasts, which are often more swollen.

At the beginning of pregnancy, a hormone called human chorionic gonadotropin increases its concentration greatly. It is the one most related to nausea and vomiting. In fact, its concentration determines the intensity of the symptoms.

It is curious because, in multiple pregnancies, this hormone increases more than in those of a single embryo. That is why they are usually more symptomatic pregnancies.

Another common sign in early pregnancy is an increased frequency and need to urinate. It is also usual for the woman to be more tired.

Positive in the test after fertilization

Currently there are different methods to confirm the presence or not of a pregnancy. However, all of them require that a certain time has passed after fertilization.

This is because pregnancy tests usually work by detecting whether the level of human chorionic gonadotropin in the urine has increased. This hormone increases between 6 and 14 days after fertilization.

In some cases, this substance can appear in the blood instead of the urine. This particular test cannot be done at home, but it is more accurate.

Therefore, what we must bear in mind is that until a certain time has passed since fertilization, it is impossible to determine whether or not there is a pregnancy. Also, pregnancy tests can have false negatives and positives. Therefore, whenever there is doubt, the ideal is to consult a specialist.

Fertilization is not an instantaneous process

As we have explained, fertilization is a complex process that requires optimal conditions. It does not occur at the same time that the sexual act ends. In fact, it may occur 2 or 3 days after ejaculation.

Also, even if a zygote has formed, an embryo will not always develop or a pregnancy will occur. For this to happen, it is essential that the implantation process ends properly.

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