In a human female, the reproductive phase starts on the onset of puberty and ceases around middle age of the female. Study the graph given below regarding menstrual cycle and answer the questions that follow:
\begin{figure}[h!]
\centering
\includegraphics[width=0.8\textwidth]{Screenshot 2025-01-17 101211.png} % Replace 'path_to_your_image.png' with the actual path to your graph image
\caption{Graph showing the hormonal regulation and events during the menstrual cycle.}
\label{fig:menstrual_cycle}
\end{figure}
In a normal human female, why does menstruation only occur if the released ovum is not fertilised? Explain.}