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Part a: 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 abdomen protrudes out as far as the leg bud. Part b: The second trimester fetus has long arms and legs, and is attached to the placenta, which is round and larger than the fetus. Part c: This illustration shows a third trimester fetus, which is a fully developed baby. The fetus is upside down and pressing on the cervix. The thick umbilical cord extends from the fetus’ belly to the placenta.
(a) Fetal development is shown at nine weeks gestation. (b) This fetus is just entering the second trimester, when the placenta takes over more of the functions performed as the baby develops. (c) There is rapid fetal growth during the third trimester. (credit a: modification of work by Ed Uthman; credit b: modification of work by National Museum of Health and Medicine; credit c: modification of work by Gray’s Anatomy)

During the second trimester, the fetus grows to about 30 cm (about 12 inches) ( [link] b ). 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 elimination 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. During the third trimester, the fetus grows to 3 to 4 kg (6.5–8.5 lbs.) and about 50 cm (19–20 inches) long ( [link] c ). This is the period of the most rapid growth during the pregnancy as all organ systems continue to grow and develop.

Concept in action

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

Labor is the muscular contractions to expel the fetus and placenta from the uterus. 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 the release of oxytocin from the posterior pituitary. Oxytocin causes 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.

Section summary

The reproductive structures that evolved in land animals allow males and females to mate, fertilize internally, and support the growth and development of offspring. Gametogenesis, the production of sperm (spermatogenesis) and eggs (oogenesis), takes place through the process of meiosis.

The male and female reproductive cycles are controlled by hormones released from the hypothalamus and anterior pituitary and hormones from reproductive tissues and organs. The hypothalamus monitors the need for FSH and LH production and release from the anterior pituitary. FSH and LH affect reproductive structures to cause the formation of sperm and the preparation of eggs for release and possible fertilization. In the male, FSH and LH stimulate Sertoli cells and interstitial cells of Leydig in the testes to facilitate sperm production. The Leydig cells produce testosterone, which also is responsible for the secondary sexual characteristics of males. In females, FSH and LH cause estrogen and progesterone to be produced. They regulate the female reproductive cycle, which is divided into the ovarian cycle and the menstrual cycle.

Human pregnancy begins with fertilization of an egg and proceeds through the three trimesters of gestation. The first trimester lays down the basic structures of the body, including the limb buds, heart, eyes, and the liver. The second trimester continues the development of all of the organs and systems. The third trimester exhibits the greatest growth of the fetus and culminates in labor and delivery. The labor process has three stages (contractions, delivery of the fetus, and expulsion of the placenta), each propelled by hormones.

Art connections

[link] Which of the following statements about the male reproductive system is false?

  1. The vas deferens carries sperm from the testes to the seminal vesicles.
  2. The ejaculatory duct joins the urethra.
  3. Both the prostate and the bulbourethral glands produce components of the semen.
  4. The prostate gland is located in the testes.

[link] D

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[link] Which of the following statements about hormone regulation of the female reproductive cycle is false?

  1. LH and FSH are produced in the pituitary, and estrogen and progesterone are produced in the ovaries.
  2. Estradiol and progesterone secreted from the corpus luteum cause the endometrium to thicken.
  3. Both progesterone and estrogen are produced by the follicles.
  4. Secretion of GnRH by the hypothalamus is inhibited by low levels of estrogen but stimulated by high levels of estrogen.

[link] C

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Questions & Answers

What are eukaryotic cells?
Nwosueke Reply
where does the cell get energy for active transport processes?
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what is synapsis
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how many turns are required to make a molecule of sucrose in Calvin cycle
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why Calvin cycle occurs in stroma
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why do humans enhale oxygen and exhale carbondioxide?
Maryam Reply
why do humans enhale oxygen and exhale carbondioxide? For the purpose of breaking down the food
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process of protein synthesis
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what is cell
Zulf Reply
what is cytoplasm
uzoka Reply
cytoplasm is fluid of cell.
Deepak
how many major types of Cloning
Saeed Reply
two
amir
two
Zulf
comparative anatomy of gymnosperms?
Meenakshi Reply
anatomy of gymnosperms
Meenakshi
how genes are regulated
Ainjue Reply
what is storage of glycogen
Student Reply
glycogen is a protein content
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how many times breathing a day normally does a person have
Vernalyn Reply
100
Aadil
on average 18000 times a day when resting.
gagan
the -10 and -35 regions of prokaryotic promoters are called consensus sequences because
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Source:  OpenStax, Concepts of biology. OpenStax CNX. Feb 29, 2016 Download for free at http://cnx.org/content/col11487/1.9
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