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What happens during fertilization?

John K. Jain, MD
OBGYN (Obstetrics & Gynecology)
During fertilization sperm burrow through the egg shell (zona pellucida) to reach the outer membrane of the egg. Sperm then release enzymes that allow it to fuse with the egg membrane and deposit a pouch of chromosomes into the egg proper. Once one sperm penetrates the egg, a reaction occurs that hardens the egg membrane in order to make it impermeable to other sperm. The pouch of chromosomes from the successful sperm then join an equal number of chromosomes from the egg to create a either a male 46XY or a female 46XX zygote (early embryo).

During fertilization the nuclei of the ovum and the sperm swell, approach and ultimately come together. At this moment the nuclear membrane of the haploid gametes disappears, the chromosomes find their mate, the diploid chromosome number is restored and the zygote is created. At this moment the biological gender of the zygote is determined.

Dr. Michael Roizen, MD
Internal Medicine
After the egg drops from the ovary, it travels through the fallopian tube, where there's about a 24-hour window when it can be fertilized. Even if you try to summit Everest, this is the most dangerous journey you will ever take. If all goes right, the sperm meets the egg in the fallopian tube and the two half-genomes unite to form a complete set of genes containing all of the DNA necessary to make a new human being. The fertilized egg says thank you very much and moves along to the uterus. There, it will attach to the uterine lining and begin the amazing process of becoming a baby. Once the sperm fertilizes the egg, the fragile blastocyst multiplies its cells and must implant on the uterus wall to endure the 280-day pregnancy.

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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.