Implantation, Pregnancy and Amniotic Fluid

Challenges to clear

• Step through the whole of development from cleavage to birth.
• Reach the process that produces the state of pregnancy.
• Reach the stage at which an embryo is renamed a foetus.

The task

Follow the zygote from its first division to birth.

What this lesson covers

• Cleavage — The fertilised egg soon starts dividing into 2, 4, 8, 16 cells and so on as it travels down the oviduct.
• Morula — Repeated division of the zygote gives a solid spherical mass of cells called the morula.
• Blastocyst — By the time it reaches the uterus it is a hollow ball of numerous cells: a surrounding single cellular layer called the trophoblast, with an inner cell mass projecting centrally.
• Implantation — The blastocyst forms a pit in the endometrial lining of the uterus and gets fixed in it about 5 to 7 days after ovulation. This process is implantation, and it produces the state of pregnancy.
• Membranes form — The implanted blastocyst develops extraembryonic membranes: amnion, chorion, allantois and placenta. They stay right up to the time of birth.
• Amniotic fluid — Fluid fills the space between the amnion and the embryo. It protects against jerks and mechanical shocks, keeps an even pressure all around, allows the foetus restricted movement and prevents it sticking to the amnion.
• Five weeks — By the end of five weeks the embryo is quite advanced: the heart and blood vessels have formed, but there are no limbs yet.
• Foetus at eight weeks — Limbs have appeared and the embryo begins to have the look of a baby. From about seven to eight weeks onwards it is called a foetus.
• Birth — Gestation, the full term of development in the uterus, lasts about 280 days. The infant is born at the end of nearly 40 weeks.

Hint

Fertilisation makes the zygote; implantation fixes the blastocyst into the endometrium. It is implantation, not fertilisation, that begins pregnancy. Remember the ladder: egg, zygote, morula, blastocyst, embryo, foetus, infant.