[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Histology](https://durgical.com/en/histology/) › [Reproductive system](https://durgical.com/en/histology/#a-reproductive) › Fertilisation, implantation and the decidua

[Histology](https://durgical.com/en/histology/) · [Reproductive system](https://durgical.com/en/histology/#a-reproductive)

# Fertilisation, implantation and the decidua

- Process

How a sperm and an egg meet in the uterine tube, and how the early embryo travels to the uterus and burrows into its lining.

Updated 2026-11-01

**See it in 3D:** [Uterine tubes](https://durgical.com/en/anatomy/uterine-tube/) · [Endometrium](https://durgical.com/en/anatomy/endometrium/) · [Ovaries](https://durgical.com/en/anatomy/ovary/)

## Key points

- Fertilisation: ampulla of the uterine tube, within about 24 hours of ovulation.
- The oocyte finishes meiosis II only when fertilised.
- Cortical reaction blocks polyspermy.
- Implantation begins at about day 6–7 in the posterior upper uterine wall; syncytiotrophoblast makes hCG.
- Decidua basalis becomes the maternal placenta; its absence causes placenta accreta.
- Most ectopic pregnancies are tubal, ampulla most often.

## What it is

Fertilisation usually happens in the ampulla of the uterine tube, within about a day of ovulation. The zygote divides as cilia and muscle move it toward the uterus, arriving as a morula, and implants as a blastocyst at the end of the first week. The trophoblast invades the endometrium, which turns into the decidua; implantation outside the uterine cavity, most often in the tube, is an ectopic pregnancy.

## Structure

- Uterine tube mucosa: folded, with ciliated cells and secretory (peg) cells; muscle and cilia move the embryo.
- Blastocyst: an outer layer of trophoblast, an inner cell mass at one pole (embryoblast) and the blastocoel.
- Syncytiotrophoblast: a multinucleated layer without cell borders that erodes the endometrium.
- Decidual cells: large pale polygonal cells derived from endometrial fibroblasts.

## Step by step

- Capacitation: in the female tract, sperm lose cholesterol and coating proteins and become able to fertilise.
- Sperm pass the corona radiata and bind the zona pellucida, which triggers the acrosome reaction.
- One sperm fuses with the oocyte membrane; a fast depolarisation, then the cortical reaction, harden the zona and block polyspermy.
- The secondary oocyte, arrested in metaphase II, completes meiosis II and releases the second polar body.
- The two pronuclei meet and form a diploid zygote, which cleaves as it travels down the tube.
- Around day 3 a morula enters the uterus; it becomes a blastocyst with inner cell mass, trophoblast and a fluid cavity.
- The blastocyst hatches from the zona and attaches to the endometrium at about day 6–7 (end of week 1).
- Trophoblast splits into cytotrophoblast and an invasive syncytiotrophoblast, which secretes hCG to keep the corpus luteum alive.
- Endometrial stromal cells swell with glycogen and lipid (decidual reaction); implantation is complete during week 2.

## The three parts of the decidua

| Part | Where | Fate |
| --- | --- | --- |
| Decidua basalis | Between the embryo and the myometrium | Maternal part of the placenta |
| Decidua capsularis | Over the embryo, facing the uterine cavity | Thins and fuses with the parietalis as the sac grows |
| Decidua parietalis | The rest of the uterine lining | Fuses with the capsularis by the end of the first trimester |

## Clinical notes

- Ectopic pregnancy: tubal scarring from chlamydial or gonococcal pelvic inflammatory disease, prior tubal surgery or a past ectopic slows the embryo; the tube cannot expand and may rupture.
- In a tubal pregnancy the tubal stroma can form decidual cells, and the uterine endometrium may also show a decidual reaction without chorionic villi.
- Placenta accreta: absent decidua basalis lets villi attach directly to myometrium, often over a previous caesarean scar.
- hCG appears in maternal blood and urine soon after implantation; it is the basis of pregnancy tests.

## Memory aid

Tube to «home» in a week: day 1 zygote, day 3 morula in the uterus, day 7 the blastocyst knocks and moves in.

## Test yourself

Think of the answer, then tap the question to see it.

## Where does fertilisation usually happen?

In the ampulla, the widest part of the uterine tube.

## What event prevents more than one sperm from fertilising the oocyte?

A fast membrane depolarisation and then the cortical reaction: granules release enzymes that harden the zona pellucida and destroy sperm receptors.

## Which layer of the trophoblast invades the endometrium and makes hCG?

The syncytiotrophoblast, a multinucleated layer that forms from fusing cytotrophoblast cells.

## What does the decidua basalis become?

The maternal part of the placenta.

## Why can scarring of the uterine tube cause an ectopic pregnancy?

Damaged cilia and folds slow the embryo, so the blastocyst is still in the tube when it is ready to implant.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/histology/fertilisation-implantation-and-the-decidua/)

## See it in 3D

The structures of Durgical's 3D anatomy where it is found.

- [Uterine tubes](https://durgical.com/en/anatomy/uterine-tube/)
- [Endometrium](https://durgical.com/en/anatomy/endometrium/)
- [Ovaries](https://durgical.com/en/anatomy/ovary/)

## Related

- [Uterine tube](https://durgical.com/en/histology/uterine-tube/)
- [Uterus and the endometrial cycle](https://durgical.com/en/histology/uterus-and-the-endometrial-cycle/)
- [Ovary: follicles, ovulation and corpus luteum](https://durgical.com/en/histology/ovary-follicles-ovulation-and-corpus-luteum/)
- [Placenta](https://durgical.com/en/histology/placenta/)
- [Cell cycle, mitosis and meiosis](https://durgical.com/en/histology/cell-cycle-mitosis-and-meiosis/)
- [Stem cells and tissue renewal: labile, stable and permanent tissues](https://durgical.com/en/histology/stem-cells-and-tissue-renewal-labile-stable-and-permanent-tissues/)

## Related conditions

- [Ectopic pregnancy](https://durgical.com/en/conditions/ectopic-pregnancy/)
- [Molar pregnancy (hydatidiform mole)](https://durgical.com/en/conditions/molar-pregnancy/)
- [Endometriosis](https://durgical.com/en/conditions/endometriosis/)
- [Placenta praevia](https://durgical.com/en/conditions/placenta-praevia/)

## Microorganisms

- [Chlamydia trachomatis](https://durgical.com/en/microbiology/chlamydia-trachomatis/)
- [Neisseria gonorrhoeae](https://durgical.com/en/microbiology/neisseria-gonorrhoeae/)

[← All of histology](https://durgical.com/en/histology/)

Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.

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: MedlinePlus Medical Encyclopedia](https://medlineplus.gov/ency/article/000895.htm) (MedlinePlus, US National Library of Medicine, 2026).

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Representative model, for educational purposes only.

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Source: https://durgical.com/en/histology/fertilisation-implantation-and-the-decidua/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/histology/fertilisation-implantation-and-the-decidua/