How Many Days After Fertilization Does Implantation Occur ?

April 11, 2020

Fertilization Process

Mentor Name: Dr. Kanika on April 11, 2020

Fertilization-

Fertilization is a complex process where a sperm and egg unite together to form a zygote(fertilized egg). Thus the zygote carries the genetic material from both the sperm and egg. It occurs in the ampullary part of the fallopian tube .

The ovum or egg immediately after ovulation from the ovary is picked up by fimbriae of fallopian tube which partly envelope the ovary mainly at the time of ovulation. Then the ovum is transported to the ampullary part of

fallopian tube.

After sexual intercourse and ejaculation, sperms deposited in the vagina travel through the cervix, uterine cavity to reach the fallopian tube by swimming with the help of its tail. The tubal transport is also facilitated by the muscular contraction of fallopian tube.

The fertilizable lifespan of an unfertilised egg ranges from 12 to 24 hours and that of a sperm is 48 to 72 hours approximately. If a sperm penetrates an egg inside the fallopian tube the egg changes its zona pellucida(outer covering of an egg) instantly to prevent other sperms from entering.

Preparation of uterus for pregnancy-

During menstruation the uterus sheds the endometrium which was built up in the previous menstrual cycle. After shedding, the endometrium responds to oestrogen hormone that is produced in the ovaries. Oestrogen causes the endometrial lining of the uterus to thicken by stimulating the endometrial cells to divide. After ovulation the endometrium prepares for the possible arrival of a fertilized egg in response to increased level of progesterone hormone that is secreted from the ovary to increase the endometrial receptivity.

Morula-

After the completion of fertilization process and formation of zygote which is a single cell structure there cell division occurs resulting in formation of a 2-cell embryo. This division continues and the embryo growth progresses to 4 cell stage on Day-2, 8- cell stage on Day-3, 16 cell stage on Day-4 until a cluster of cells called morula is formed which resembles a mulberry.The morula then enters the uterine cavity from the fallopian tube usually on the fourth day of fertilization. The transport is a slow process and is controlled by the muscular contraction and movement of the cilia of fallopian tube.

The central cells of the morula are known as inner cell mass which forms the embryo proper and the peripheral cells are called the outer cell mass which are the protective and nutritive membranes of embryo.

Blastocyst-

The morula remains free inside the uterine cavity usually on the fourth and fifth day of fertilization. The morula absorbs fluid through its covering which causes separation of cells of morula to become blastocyst. So approximately 5 to 6 days after the fertilization process is over the embryo reaches the stage of blastocyst.

Zona hatching-

Due to enlargement of blastocyst its outer covering which is called zona pellucida becomes stretched and thin and gradually disappears and escape of embryo occurs. The uterine enzymes also help in breakdown of the zona pellucida.
This is called zona hatching.

The cells on the outer side of morula become trophectoderm which differentiates into chorion which forms the future placenta and the inner cell mass become the embryo.

Implantation-

Implantation is the process where the blastocyst attaches and burrows into the endometrium (the inner lining of uterine wall). Implantation usually occurs in the body of uterus in the functional layer of endometrium usually on the 5th to 6th day after fertilization. After zona hatching the trophectoderm cells interact with endometrial cells and implantation occurs. It occurs through many stages i.e. apposition, adhesion, penetration, and invasion.

Successful implantation requires a competent blastocyst and receptive endometrium during a specific window of time during the menstrual cycle i.e. day 20 to day 24 of a regular cycle which is day 6 to day 10 of fertilized egg for a successful pregnancy. The establishment of pregnancy is not completed until the fertilized egg has implanted in the endometrial lining of uterus.

About 1/3rd women have slight bleeding during implantation called as implantation bleeding which is much lighter than the regular menstrual bleeding. Some women may feel cramping pain during implantation process.
If implantation does not take place the fertilized egg will leave the body and pregnancy will not occur. Implantation failure is responsible for significant pregnancy loss.

Once implantation is completed the cervix becomes closed with mucus plug. After successful implantation pregnancy can be confirmed by pregnancy test as the body starts to produce hormone HCG.

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