Final answer:
Fertilization is the process where a sperm and an egg combine in the fallopian tube to form a zygote, which is the first stage of pregnancy. This process is influenced by various biological factors like sperm survival and the egg's lifespan. Implantation follows fertilization and happens when the embryo attaches to the uterus lining.
Step-by-step explanation:
Fertilization and Implantation
Fertilization occurs when a sperm and an oocyte (egg) combine and their nuclei fuse, creating a zygote with a complete set of genetic material, half from each parent. This usually happens in the fallopian tube, where the sperm will overcome various challenges such as vaginal acidity and cervical mucus to reach the egg. The zygote then undergoes several cell divisions before it is ready for the next stage, implantation, which involves the embryo attaching itself to the uterus lining.
Obstacles Sperm Must Overcome
During ejaculation, hundreds of millions of sperm are released, but only a few thousand will make it to the oocyte due to the vagina's acidity, cervical mucus, uterine leukocytes, and the long journey to the fallopian tubes. Sperm that do not immediately encounter an egg can live in the uterine tubes for 3–5 days, giving a window for fertilization if intercourse occurs a few days before ovulation, as the oocyte can only survive for approximately 24 hours post-ovulation.