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Fertilisation Explained: The Process of Sperm Meeting Egg Naturally or Through IVF

Reviewed by Dr. Manisha Acharya ,

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Last updated: November 10, 2025

Synopsis

Hearing positive pregnancy news is one of the happiest moments in the lives of couples waiting to embark on their parenthood journey. But pregnancy is just the result of a series of complex procedures that happen inside the body. Medically, when two cells, an egg and a sperm, meet inside the body, it is termed fertilisation. This is the first step towards pregnancy. Fertilisation can happen either naturally or through IVF. But as simple as the process may sound, it is a highly coordinated biological process that needs proper understanding, and this article covers every aspect of this complex process. So, let’s begin!

What Is Fertilisation?

Fertilisation is the fusion of a male sperm cell and a female egg cell, which results in the first cell of a new life — the zygote. A zygote is the blueprint of the genetic material (DNA) of both parents. In the natural way of conception, fertilisation occurs in the fallopian tube within 12 to 24 hours of ovulation. However, in IVF, the fertilisation is done with precise tools in a laboratory setting.

Once the egg is fertilised and turns into a zygote, it starts to divide and move to the uterus for implantation. This process marks the beginning of a probable pregnancy, whether in the fallopian tube or a laboratory.

Natural Fertilisation: Step-by-Step Process

A successful fertilisation requires coordinated planning, starting with tracking the menstrual cycle. Let’s take a look at how fertilisation happens, step-by-step:

Ovulation:

Ovulation is the release of a mature egg cell into the fallopian tube by one of the two ovaries. Typically, this happens exactly 14 days before the start of the period. Released one per month, this egg cell remains viable for 12 to 24 hours.
Sperm Journey:

Millions of sperm are released into the vagina during intercourse, but only the strongest and healthiest sperm survive the uterus’s hostile conditions. These sperm swim across the cervix to the uterus, then to the fallopian tubes, among which only one sperm (in most cases) is selected by the egg cell to fertilise it.
Fertilisation in the Fallopian Tube:

Once a sperm penetrates the outer membrane of the egg, the nuclei fuse; the fusion creates a zygote with a full set of chromosomes, that is, 46, if normal.
Zygote to Embryo:

The single-celled zygote now divides continuously into multiple cells, leading to the formation of an embryo. After about 3 to 5 days of fertilisation, the embryo, after travelling down the uterus, implants into the uterine lining, from where the pregnancy officially begins.

Fertilisation in IVF: How It Happens in the Lab

IVF (In vitro fertilisation), one of the most effective ART techniques, can enhance the chances of fertilisation and eventually result in a successful pregnancy. However, the IVF process follows a carefully guided and monitored lab protocol and steps, which include:

Egg Retrieval: This is the first step, in which medications are provided to females to stimulate the ovaries and produce, not just one but multiple eggs. These eggs are then retrieved and collected using a specialised tool under anaesthesia.
Sperm Collection and Preparation: The semen sample from the male partner or a donor is collected, washed and concentrated to choose only the healthiest and best ones.
Fertilisation Methods: While the purpose remains the same, fertilisation in IVF is done through two methods:

1. Conventional IVF: In this method, eggs and sperm are placed in a petri dish to allow them to fertilise naturally.

2. ICSI (Intracytoplasmic Sperm Injection): In case of low sperm count or motility, ICSI may also be used. In this, a single sperm is directly injected into an egg to initiate fertilisation.
Zygote Formation: Once fertilisation is done, the zygote is monitored in a laboratory setting for about 3 to 5 days before being transferred to the uterus.

Key Differences: Natural vs IVF Fertilisation

Aspect	Natural Fertilisation	IVF Fertilisation
Location	In this, the sperm meets the egg in the fallopian tubes	While in IVF, this happens in the specialised laboratories
Sperm-Egg Interaction	Naturally through intercourse	Assisted by embryologists
Sperm Quantity Needed	Millions of sperm compete to fertilise one egg.	Few sperm are used, sometimes only one in ICSI.
Monitoring	Not visible, so it can not be tracked precisely.	Done in a laboratory setting, hence it can be monitored and tracked at every phase.

What Happens After Fertilisation?

Whether in the laboratory or naturally in the womb, fertilisation follows the same approach, which includes:

Division of the zygote, which is single-celled, into multiple cells, forming an embryo.
In case of natural fertilisation, the embryo travels down the fallopian tube into the uterus.
When IVF is used, the embryo is directly transferred into the uterus via specialised tools by an embryologist.
Implantation usually takes 6 to 10 days post-fertilisation. The increased hCG hormone levels confirm pregnancy.

Factors That Affect Fertilisation Success:

Know that fertilisation is not a standalone event; it is affected by several factors that can affect the trajectory of its success. These are:

Age: The fertility of a female usually declines after 35, which eventually leads to multiple unsuccessful pregnancies and fertilisation not occurring.
Egg and sperm quality: Decreased quality of egg and sperm due to various factors can also bring down the chances of fertilisation.
Timing of Intercourse: Although sperm can survive in the woman’s uterus for 5 to 6 days, egg cells stay viable for only 12 to 24 hours, making it essential to have planned intercourse just around the time of ovulation.
Hormonal Balance: Hormonal imbalance can disrupt the natural timing of ovulation and menstruation, which in turn decreases the success rate of fertilisation.
Underlying Fertility Conditions: Fertility conditions such as endometriosis, PCOS, or thyroid issues can also interfere with natural fertilisation.

Conclusion:

So, what is Fertilisation? In simple terms, it is the critical first step in human reproduction; it is the moment when a sperm meets an egg to create a new life. It can happen naturally in the body or through assisted technologies, such as IVF. Though different, both procedures attempt to reach the same goal: conception. This is why understanding the process of fertilisation, whether natural or IVF, enables individuals and couples to make better fertility decisions as per their needs.