Fertilisation typically occurs in the fallopian tube; however, in rare cases, it can occur outside the tube, leading to ectopic pregnancy. If fertilisation happens outside the fallopian tube, usually in the abdominal cavity or cervix, it can cause significant health risks. In an ectopic pregnancy, the embryo implants and starts growing in a location not suited for pregnancy, often leading to complications. The most common type of ectopic pregnancy is a tubal pregnancy, where the embryo implants in the fallopian tube. As the embryo grows, it can cause the tube to rupture, leading to severe internal bleeding and posing a serious health risk to the mother. Ectopic pregnancies are not viable and require immediate medical intervention. Understanding the precise location of normal fertilisation is crucial, as it highlights the importance of the correct environment for embryo development and the potential complications arising from deviations in this process.

Chemical attractants, released by the egg, play a crucial role in guiding sperm to the egg in a process known as chemotaxis. These chemical signals are essential for ensuring that sperm swim in the right direction, increasing the likelihood of successful fertilisation. The egg releases specific molecules that create a chemical gradient in the surrounding environment. Sperm cells detect this gradient through receptors on their surface and respond by altering their swimming pattern, moving towards higher concentrations of the attractants. This chemotactic navigation is vital in the vast and complex environment of the female reproductive tract, where the likelihood of randomly encountering an egg is low. It ensures that sperm expend their energy efficiently, maximising the chances of reaching the egg. The precision of this chemical guidance system highlights the intricacy and fine-tuning of reproductive processes in ensuring successful fertilisation.

The timing of ovulation is crucial for the success rate of fertilisation. Ovulation is the process where an ovary releases an egg into the fallopian tube. For fertilisation to occur, sperm must meet the egg during its journey towards the uterus, which is a time-sensitive process. The egg remains viable for fertilisation for about 12 to 24 hours after ovulation. Therefore, for fertilisation to be successful, sperm must be present in the fallopian tube within this timeframe. Sperm can survive in the female reproductive tract for up to five days, so intercourse a few days before ovulation can still lead to successful fertilisation. The synchronisation between sperm presence and ovulation is essential, as fertilisation is unlikely to occur if the timing is not aligned. This interplay highlights the importance of understanding reproductive cycles and timings in the context of fertility and conception planning.

The zona pellucida, a glycoprotein layer surrounding the egg, plays multiple essential roles in fertilisation. Initially, it serves as a barrier that selectively binds to sperm. Upon sperm binding, the zona pellucida induces the acrosome reaction in the sperm, facilitating the release of enzymes necessary for penetrating the egg's outer layers. This selective binding ensures that only sperm of the same species can fertilise the egg, maintaining species-specific reproduction. After the sperm penetrates the zona pellucida and fertilises the egg, the zona undergoes a hardening process, preventing other sperm from binding and entering the egg (polyspermy). This hardening is a result of enzymatic changes triggered by the cortical reaction, ensuring the correct chromosomal makeup of the zygote. Thus, the zona pellucida's functions are crucial for both the initial binding of the sperm to the egg and the prevention of polyspermy.

Capacitation is a vital physiological change that sperm undergo after entering the female reproductive tract. This process is significant as it enhances the sperm's ability to penetrate the egg, a prerequisite for successful fertilisation. During capacitation, changes occur in the sperm's plasma membrane, making it more fluid and permeable. This alteration facilitates the release of enzymes from the acrosome in the acrosome reaction. Furthermore, capacitation activates the sperm's motility, increasing its movement and ability to swim towards the egg. It also involves the removal of glycoproteins and seminal plasma proteins that initially coat the sperm, thus allowing it to recognise and bind to the egg. Without capacitation, the sperm would be unable to effectively bind and penetrate the egg, making it an indispensable step in the fertilisation process.