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Photo shows a human fetus, with a large bent head and a dark eye, fingers on its arm and a leg bud. The spine is visible through the back, and the stomach protrudes out as far as the leg bud.
Fetal development is shown at nine weeks gestation. (credit: Ed Uthman)

During the second trimester, the fetus grows to about 30 cm (12 inches), as shown in [link] . It becomes active and the mother usually feels the first movements. All organs and structures continue to develop. The placenta has taken over the functions of nutrition and waste and the production of estrogen and progesterone from the corpus luteum, which has degenerated. The placenta will continue functioning up through the delivery of the baby.

The second trimester fetus has long arms and legs, and is attached to the placenta, which is round and larger than the fetus.
This fetus is just entering the second trimester, when the placenta takes over more of the functions performed as the baby develops. (credit: National Museum of Health and Medicine)

During the third trimester, the fetus grows to 3 to 4 kg (6 ½ -8 ½ lbs.) and about 50 cm (19-20 inches) long, as illustrated in [link] . This is the period of the most rapid growth during the pregnancy. Organ development continues to birth (and some systems, such as the nervous system and liver, continue to develop after birth). The mother will be at her most uncomfortable during this trimester. She may urinate frequently due to pressure on the bladder from the fetus. There may also be intestinal blockage and circulatory problems, especially in her legs. Clots may form in her legs due to pressure from the fetus on returning veins as they enter the abdominal cavity.

Illustration shows a third trimester fetus, which is a fully developed baby. The fetus is up-side down and pressing on the cervix. The thick umbilical cord extends from the fetus’s belly to the uterine wall.
There is rapid fetal growth during the third trimester. (credit: modification of work by Gray’s Anatomy)

Visit this site to see the stages of human fetal development.

Labor and birth

Labor is the physical efforts of expulsion of the fetus and the placenta from the uterus during birth (parturition). Toward the end of the third trimester, estrogen causes receptors on the uterine wall to develop and bind the hormone oxytocin. At this time, the baby reorients, facing forward and down with the back or crown of the head engaging the cervix (uterine opening). This causes the cervix to stretch and nerve impulses are sent to the hypothalamus, which signals for the release of oxytocin from the posterior pituitary. The oxytocin causes the smooth muscle in the uterine wall to contract. At the same time, the placenta releases prostaglandins into the uterus, increasing the contractions. A positive feedback relay occurs between the uterus, hypothalamus, and the posterior pituitary to assure an adequate supply of oxytocin. As more smooth muscle cells are recruited, the contractions increase in intensity and force.

There are three stages to labor. During stage one, the cervix thins and dilates. This is necessary for the baby and placenta to be expelled during birth. The cervix will eventually dilate to about 10 cm. During stage two, the baby is expelled from the uterus. The uterus contracts and the mother pushes as she compresses her abdominal muscles to aid the delivery. The last stage is the passage of the placenta after the baby has been born and the organ has completely disengaged from the uterine wall. If labor should stop before stage two is reached, synthetic oxytocin, known as Pitocin, can be administered to restart and maintain labor.

Questions & Answers

why some kinds of students are failed
Ahmadi Reply
lack of concentration
Faith
lack of guidance and counseling
ali
What is the meaning of optic
Kisaky Reply
Giving a specific section of the alimentary canal,describe 3 ways in which physical digestion occurs.
Kisaky
mouth when chewing
ephraim
what is population
Ivy Reply
total number of people living in an area
FILDA
a number of people lives in one catigorize area or named area
Oburak
what is a cell
Chiko Reply
basic and functional unit of life
Edwin
cell is tissues that makes up functional life in human or un animal.
Oburak
is the smallest basic unit of life.
Kisaky
Is the smallest baic unit. o
Kisaky
why cell is very important to human body
Ahmadi
what is soil
FILDA Reply
Is the finely divided material covering the earth crust.
Kisaky
is the upper moist of layer of the earth surface
Ahmadi
what is reducing sugar
Erica Reply
in genetics which disease is also termed as the queen disease
Phinihas Reply
what are the types of cell
Teye Reply
prokaryote ,eukaryote, akaryotes
bonney
what is akaryotes ?
Chriscia
multicellular and unicellular
Edwin
akaryotes are organisms that function as eukaryotes and prokaryotes
Edwin
akaryotes are cell with no nucleus
Edwin
biology is the study of living organisms
bonney Reply
what are the important of cells in the body
Nharnhar Reply
what is biology
Saidu Reply
Is the study of living things
Nharnhar
it is the scientific study of living organisms
Kenneth
ls the study of life and living organisms
Charles
what are the impotance of proteins
Mark
what is the standArd deviation
mascuud
is the study of living organisms
Erica
what is diffusion
Lenox Reply
what are the important of ecology?
Lenox
Is the movement of molecules of gasses and liquids from the region of high concentration to the region of low concentration down the concentration grandient
Kisaky
what are the importance of ecology
Foday Reply
double stranded DNA is found in which viruses?
Deborah Reply
Virusws usually dont hsve double strnaded DNA they have a single strand RNA. U should probably check them in gpogle just to be sure
Danisha
what would happen if humans were not multicellular
Grace Reply
ettr
Grace
sorry but no
Grace
what is biology
Emmanuel
study of life
Dads
what are types of photosynthesis
Dads

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Source:  OpenStax, Biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11448/1.10
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