Gestation (pregnancy) (6) Flashcards
What is gestation?
Gestation is the time between conception (fertilisation) and birth, during which the embryo and then the foetus develops in the uterus. Usually this is about 280 days (about 40 weeks).
What are the two phases of prenatal development?
Early development and later development.
What does early development (0 to end of 2 months) entail? (3)
a.Fertilisation b.Blastocyst and implantation formation c.Embryo formation
What does later development (3 months to birth) entail?
the foetal stage
Where does fertilisation occur?
Fertilisation occurs in the fallopian tube when the nuclei of the sperm and egg fuse to form a zygote.
How long does the sperm survive?
A sperm lives for about 48 hours and an egg approximately 24 hours. If fertilisation does not occur within this period the gametes will degenerate.
How does semen reach the egg?
After semen is deposited in the vagina, sperm move through the cervix, uterus and into the fallopian tubes until they reach the egg. The egg, surrounded by follicle cells, is sucked into fallopian tube by the action of the fimbriae
Fertilisation usually occurs in the top part (outer third) of the ____ tube.
Fertilization
How does fertilisation occur? (5)
−Thousands of sperm surround the egg.
− Hydrolytic enzymes, released from the acrosomes, break down the follicle cells.
− Only one sperm penetrates the membrane of the egg; the tail is discarded.
−The nuclei of the sperm and egg fuse forming a diploid zygote.
−A fertilisation (egg) membrane forms immediately, which prevents other sperm from entering the egg
How does a blastocyst form? (5)
After fertilisation:
• The diploid zygote divides immediately by mitosis to form two cells.
• Each of these divides repeatedly until a solid ball of cells, the morula (resembles a mulberry) is formed. −The morula develops into a hollow, fluid-filled ball of cells, the blastocyst.
−The outer cells forming the wall of the blastocyst form a layer known as the trophoblast.
• An inner cell mass develops from the trophoblast.
• These changes take place as the developing embryo moves along the fallopian tube towards the uterus. It moves by a combination of peristaltic waves and the rhythmic beating of the tube’s cilia.
• After a few days in the uterus the blastocyst sinks into the thickened, highly vascular endometrium. This is called implantation and occurs about 10 days after fertilisation.
Tropho=
relates to feeding
What happens after implantation? (2)
After implantation the: • inner cell mass forms the:
− embryo (the future baby)
− amnion, a membrane that becomes fluid-filled
− yolk sac, which forms part of umbilical cord
• trophoblast (later called the chorion) develops villi that grow into the endometrium to form the placenta.
The embryo develops into three layers. The outer layer forms the ____ and the inner, the ______. The ____ develops between the ecto– and endoderm.
ectoderm
endoderm
mesoderm
All the different tissues and organs of the body will form from these three layers by further cell ____ and _____. During this time all major organs appear. The heart can be seen beating. The eyes, ears and nose can be seen on the large head. Arms and legs develop and fingers and toes are visible. The embryo grows from ___ mm to ___ mm.
division
differentiation
1.5
30
The period of differentiation is a most critical period as embryological malformations that can lead to birth defects can be caused by: (3)
- drugs
- viruses, e.g. German measles virus
- environmental factors such as pesticides.
The embryonic phase lasts from the ___ week until the ____ week after fertilisation.
third
eighth
How long does the foetal period of gestation last?
The foetal period of gestation lasts from the ninth week to the end of the pregnancy. During this period some differentiation does continue, but the major change is the rapidgrowth of the body.
As the foetus approaches term (birth), bones ____, much subcutaneous adipose (fat) tissue is formed and antibodies enter the ____ via the placenta to fight disease. The foetus usually turns so that its head points downwards towards the ____.
ossify
foetus
cervix
What is an ultrasound?
An ultrasound is a type of technology that uses high-pitched sound waves that cannot be heard by the human ear. The sound is bounced off a solid structure, e.g. the embryo and foetus.
The echoes form an ultrasound or sonogram image that will enable a doctor to view a growing foetus giving information of: (2)
• foetal age, size and growth during normal development. • an early diagnoses of any complication.
What is the difference between an embryo and a foetus?
The term embryo refers to the developing zygote until 8 weeks. During this time all the organs of the body are being formed by differentiation. After the organs have developed, from nine weeks until birth, it is known as a foetus. Growth and organ specialisation takes place during this time.
