[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Reproduction, pregnancy and the fetus](https://durgical.com/en/physiology/#s-repro) › Fertilization and implantation [Physiology](https://durgical.com/en/physiology/) · [Reproduction, pregnancy and the fetus](https://durgical.com/en/physiology/#s-repro) # Fertilization and implantation - Mechanism - Exam priority: medium A sperm meets the egg in the fallopian tube; the fertilized egg divides, travels to the uterus and burrows into its lining about a week later. Updated 2026-11-01 Also called: Capacitation, Acrosome reaction, Blastocyst **See it in 3D:** [Ampulla of uterine tube](https://durgical.com/en/anatomy/ampulla-of-uterine-tube/) · [Uterine tubes](https://durgical.com/en/anatomy/uterine-tube/) · [Endometrium](https://durgical.com/en/anatomy/endometrium/) · [Body of uterus](https://durgical.com/en/anatomy/body-of-uterus/) · [Cervix](https://durgical.com/en/anatomy/cervix-of-uterus/) · [Ovaries](https://durgical.com/en/anatomy/ovary/) ## Key points - Site of fertilization: ampulla of the uterine tube; site of normal implantation: posterior wall of the uterine body. - Capacitation (hours in the female tract) removes cholesterol and glycoproteins from the sperm membrane; required for the acrosome reaction. - The cortical reaction (Ca²⁺ wave, zona hardening) blocks polyspermy. - Implantation occurs about 6–10 days after fertilization; the syncytiotrophoblast invades and secretes hCG. - Implantation outside the uterine cavity is an ectopic pregnancy, most often in the ampulla (risk: prior tubal damage, salpingitis). ## What it is Fertilization usually happens in the ampulla of the uterine tube within a day of ovulation. Sperm must first be capacitated in the female tract, then undergo the acrosome reaction to cross the zona pellucida. The zygote divides on its way to the uterus, reaches it as a morula, becomes a blastocyst, hatches and implants into the progesterone-prepared endometrium, where its trophoblast begins to make hCG. ## How it works, step by step - Sperm deposited in the vagina pass the cervix (thin mid-cycle mucus) and are capacitated in the uterus and tube. - A capacitated sperm binds the zona pellucida (ZP proteins) and releases acrosomal enzymes to digest a path. - Sperm and oocyte membranes fuse; a calcium wave triggers the cortical reaction that hardens the zona against other sperm. - The oocyte completes meiosis II; male and female pronuclei form and combine: a diploid zygote. - Cleavage divisions produce a morula (day 3–4) that enters the uterine cavity. - The blastocyst (inner cell mass and trophoblast) hatches from the zona around day 5–6. - It attaches to the receptive endometrium; the trophoblast splits into cytotrophoblast and invasive syncytiotrophoblast. - Progesterone turns the endometrial stroma into decidua, which nourishes the embryo and limits invasion; hCG maintains the corpus luteum. ## Timing | Parameter | Value | Note | | --- | --- | --- | | Oocyte fertilizable after ovulation | ≈12–24 h | | | Sperm survival in the female tract | up to ≈5 days | fertile window ≈6 days | | Arrival of the morula in the uterus | ≈day 3–4 | after fertilization | | Implantation | ≈day 6–10 | after fertilization | | hCG detectable in serum | ≈8–10 days after ovulation | | Approximate values for a healthy adult at rest; each laboratory sets its own ranges. ## Clinical notes - Ectopic pregnancy: pain, bleeding and an hCG that rises too slowly; tubal rupture causes hemoperitoneum and shock. - Implantation bleeding can be mistaken for a light period. - Spontaneous early pregnancy loss is common; most early losses are due to chromosomal abnormalities of the embryo. ## Memory aid Capacitate, Acrosome, Cortical block, Cleave, then Implant: the CAC-CI path from tube to uterus. ## Test yourself Think of the answer, then tap the question to see it. ## What blocks a second sperm from fertilizing the oocyte? Sperm fusion triggers a calcium wave that releases cortical granules; their enzymes modify the zona pellucida so it no longer binds or admits sperm. ## Why can intercourse several days before ovulation result in pregnancy? Sperm can survive in the female tract for up to about 5 days, while the oocyte lives only 12–24 h; the fertile window is the days before and the day of ovulation. ## Why can tubal scarring after pelvic inflammatory disease cause ectopic pregnancy? Damaged cilia and narrowed lumen slow the embryo's transport; it reaches the blastocyst stage and implants in the tube before arriving in the uterus. [Add to my cards (3)](https://durgical.app/estudiar/?add=/en/physiology/fertilization-and-implantation/) ## See it in 3D The structures of Durgical's 3D anatomy where it happens. - [Ampulla of uterine tube](https://durgical.com/en/anatomy/ampulla-of-uterine-tube/) - [Uterine tubes](https://durgical.com/en/anatomy/uterine-tube/) - [Endometrium](https://durgical.com/en/anatomy/endometrium/) - [Body of uterus](https://durgical.com/en/anatomy/body-of-uterus/) - [Cervix](https://durgical.com/en/anatomy/cervix-of-uterus/) - [Ovaries](https://durgical.com/en/anatomy/ovary/) ## Related topics - [Oogenesis, follicles and ovulation](https://durgical.com/en/physiology/oogenesis-follicles-and-ovulation/) - [The placenta and pregnancy hormones](https://durgical.com/en/physiology/the-placenta-and-pregnancy-hormones/) - [The menstrual cycle: ovary and endometrium](https://durgical.com/en/physiology/the-menstrual-cycle-ovary-and-endometrium/) - [Maternal physiological changes in pregnancy](https://durgical.com/en/physiology/maternal-physiological-changes-in-pregnancy/) ## Related conditions - [Ectopic pregnancy](https://durgical.com/en/conditions/ectopic-pregnancy/) - [Miscarriage (early pregnancy loss)](https://durgical.com/en/conditions/miscarriage/) [← All of physiology](https://durgical.com/en/physiology/) Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given. Sources: [28.1 Fertilization](https://openstax.org/books/anatomy-and-physiology-2e/pages/28-1-fertilization) (OpenStax (Rice University), 2022); [28.2 Embryonic Development](https://openstax.org/books/anatomy-and-physiology-2e/pages/28-2-embryonic-development/) (OpenStax (Rice University), 2022); [Ectopic Pregnancy](https://www.ncbi.nlm.nih.gov/books/NBK539860/) (StatPearls (NCBI Bookshelf), 2025); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 83). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016). ## **Spotted a mistake?** We are always improving for the community. Tell us what is wrong and our team will review it. Report a mistake Representative model, for educational purposes only. --- Source: https://durgical.com/en/physiology/fertilization-and-implantation/ License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/fertilization-and-implantation/