Implantation is the process that occurs in the female after fertilisation. Mostly, implantation takes place on the 7^th day after fertilisation. A woman is not said to be pregnant when there is successful implantation. If implantation does not occur, the thickened endometrial lining sheds off to form a woman’s menstrual period.

Implantation takes place in five stages namely, hatching, adplantation, apposition, adhesion, and invasion. The symptoms of implantation are observed after 6−10 days generally after conception. In this article, we will provide detailed information about Implantation. Read on to find out!

Define Implantation

Implantation refers to the embedding of a blastocyst in the endometrium of the uterus. It begins about the seventh day after fertilisation of the ovum, generally between \(6th\) to \(9th\) days. This process takes approximately three days to complete.

Fig: Fertilized Egg Development

What is Implantation?

After fertilisation, i.e., the fusion of male and female gametes, a female is said to be conceived and the zygote formed travels down into the uterus, which after a series of cleavage divisions becomes the morula. Typically, morula or little mulberry is a \(16\)-celled stage.

Fig: Blastocyst

Morula changes to the blastocyst, which burrows itself into the lining of the uterus and forms a connection with the maternal tissue to get nourishment for its growth and development. This process is called implantation.

Progesterone Hormone will remain high throughout the gestation period which is responsible for further thickening (up to \(5{\rm{ mm}}\)) of the endometrial lining.

Blastulation is the process of development of the morula prior to implantation.

The young embryo (morula) reaches the uterus; its outer cell begins to absorb nutrients and grow while covering the zona pellucida.

The outer cells enlarge and become flat. This forms the trophoblast or tropho-ectoderm.

The trophoblast encloses the cavity called blastocoel or blastocyst cavity, and the embryo is now termed as Blastocyst. 

The trophoblast encloses the Inner cell mass (ICM).

The trophoblast, with development, becomes two-layered, outer Syncytiotrophoblast and inner Cytotrophoblast.

This secretes human Chorionic Gonadotropin (hCG), forms villi for implantation, and in later stages forms chorion, amnion, and the foetal part of the placenta.

Fig: Implantation

Stages of Implantation

Implantation occurs in the following stages:

Hatching

a. In this stage, zona pellucida gets dissolved to let the blastocyst of this transparent layer and is replaced by trophoblastic cells.

Adplantation

a. In this stage, the blastocyst slowly rolls and aligns properly to the endometrial epithelium of the uterus.

Apposition

a. In this stage, there is a formation of a loose connection between the blastocysts and endometrium of the uterus.

Adhesion

1. When blastocysts come in contact with the endometrial lining, then local stromal oedema appears in the endometrium. This is the first sign of decidual cell reaction (DCR).

2. In this decidual cell reaction,

a. Permeability of blood vessels and wall of endometrium increases.
b. Fibroblasts become larger and get converted into glycogen-filled cells.

Invasion

1. The trophoblast (outer lining of blastocyst) starts making finger-like projections, and these penetrate into the endometrium, which gets deeper into the lining. These projections are called chorionic villi. Endometrium gets disrupted, and the chorionic villi get penetrated.
2. This penetration goes on till it gets in contact with the maternal blood. After that, the actual connection is formed.
3. Placenta, a disc-shaped organ that develops in the uterus during pregnancy through which the developing embryo gets nutrition, oxygen and removes waste materials. 
4. The placenta is attached to the developing embryo through the umbilical cord and to the maternal tissue.
5. Chorionic villi penetrate and continue to grow till it is going to touch the maternal blood, from which the embryo will get the nutrition to grow.
6. The endometrial lining grows and covers the complete blastocyst. This type of implantation is called the interstitial type. This type of implantation is found in apes and humans.
7. In this type of implantation, the blastocyst gets buried inside the endometrial tissue. By this chorionic villi-tissue connection is established.
8. Once the chorionic villi have penetrated to the maternal tissue, we say implantation has occurred.
9. Implantation causes nutrients enrichment, enlargement of cells, and the vascular endometrium. This is now called the Decidua of Pregnancy.

a. There are irregular stomal cells that accumulate glycogen and lipid in their distended cytoplasm.

10. The decidua differentiates into three parts:
a. Decidua Capsularis is the part covering the embryo
b. Decidua Basalis is the part where the embryo is attached to. It is the place where the placenta develops simultaneously.
c. Decidua Parietalis is the part that is not in contact with the embryo.

Fig: Implantation

Implantation Bleeding

1. Implantation bleeding refers to the light bleeding that may occur when there is the commencement of implantation, i.e. when there is the attachment of blastocysts to the endometrium of the uterus. This is natural and nothing to worry about.
2. But sometimes, females get confused and think of implantation bleeding as menstrual bleeding as this implantation bleeding occurs a few days before the date of the next menstruation.

Implantation Symptoms

Implantation symptoms can be observed after \(6-10\) days after conception. Some of the symptoms of implantation are:-

1. Missed periods
2. Cramps
3. Light bleeding- Blood that comes out during this period is pinkish brown-red which is lighter than the menstruation blood.
4. Nausea
5. Mood swings
6. Headaches
7. Tender breasts
8. Bloating
9. Constipation

Summary

After fertilisation, implantation occurs, which refers to the attachment of the embryo to the endometrium of the uterus. This implantation is done to get the nourishment from the mother’s blood for the proper growth and development of the embryo. Implantation occurs in the following stages, i.e. hatching, plantation, apposition, adhesion, and invasion. Implantation symptoms can be observed after \(6-10\) days after conception. Like missed periods, cramps, light bleeding, nausea, mood swings, headaches, etc.

FAQs on Implantation

Q.1. What are the first signs of implantation?
Ans: Following symptoms could be the first sign of the implantation-
1. Missed periods
2. Cramps
3. Light bleeding
4. Nausea
5. Mood swings
6. Headaches
7. Tender breasts

Q.2. How many days after conception is implantation?
Ans: After 6-7 days of conception, implantation occurs.

Q.3. How does implantation feel?
Ans: During implantation, there is a feeling of cramping, back pain, nausea, headache, tender breasts, etc. This feeling is like the way a female feels before menstruation.

Q.4. Are you pregnant before implantation?
Ans: No, a female is not pregnant before implantation. Pregnancy does not mean fertilisation or conception. Until implantation, i.e. embedding of the embryo to the mother’s womb, a woman is not said to be pregnant.

Q.5. What should you not do during implantation?
Ans: During implantation, one should not do the following things:
1. Do strenuous exercise
2. Drink alcohol
3. Eat junk food.

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