Human Conception - Pregnancy

Fertilization (also called conception) in humans happens when the sperm cell reaches the egg cell and inserts its DNA into the egg cell. The physical act is called "intercourse".

During intercourse (in humans) about 150 to 300 million sperm cells are released and travel through the vagina into the uterus. Only a few hundred of these actually make it to the egg. Only a single sperm cell can actually enter the egg cell to fertilize it.

Human Conception (fertilization and implantation):

Early Stages of Pregnancy:

Step #1: Fertilization (mating à the egg and sperm unite to form the zygote)
Step #2: Cleavage (rapid cell division in first 24-48 hours; no growth)
Step #3: Blastocyst forms (hollow ball of cells with inner and outer layers)
Step #4: Gastrulation (arrangement of cells into 3 germ layers)
Step #5: Implantation (zygote attaches to uterus wall – officially an embryo)

Within a few hours of conception, tiny hair-like structures called cilia (found in the oviduct) push the zygote towards the uterus. During this journey to the uterus, the zygote starts the process of cell division.

At around 4 days, the zygote is a 16-cell mass and it enters the uterus where it floats freely for about 2 days.

Cleavage is the term used for the rapid division of cells that happens in the first 24-48 hours after fertilization. There is little/no growth in the size of the zygote.


cleavage results in the formation of a blastocyst

At around 6 days, the zygote has become mass of more than 100 cells called a Blastocyst and it has the shape of a hollow ball with inner and outer layers.
The out layer will become the placenta and the inner layer will become the fetus.

Gastrulation is the arrangement of cells into 3 distinct germ layers.


The zygote implants (attaches) to the wall of the endometrium at around 10 days. After implantation, the zygote is officially known as an embryo.

Rapid growth of the embryo continues and a yolk sac develops beside it.
This sac contains nutrients that help the embryo grow in the early stages before the umbilical cord. The embryo is called a fetus once bone cells develop.

Next, a membrane called the amnion forms around the embryo and fills with a liquid called amniotic fluid. The amniotic sac surrounds the embryo protecting it from infection, dehydration, impact and changes in temperature.

Specialized cells in the embryo combine with cells in the endometrium to form the placenta. The placenta is responsible for exchanging nutrients and waste products.

 

Key Events During Pregnancy

1st Trimester: (1-12 weeks)

  • limbs, eyes and spine begin to form
  • all major organs have begun to form
  • fetus is up to 100mm in length
  • the sex can be determined

2nd Trimester: (12-24 weeks)

  • placenta moves to one side
  • the skeleton begins to form
  • the brain grows rapidly
  • the nervous system begins to function
  • fetus is up to 300mm in length

3rd Trimester: (24-38 weeks)

  • size increases rapidly
  • the immune system develops
  • eyes open
  • typically weighs between 2700g and 4000g
  • fetus is up to 500mm in length