Pregnancy and birth

Explain what fertilisation is, in which part of the female reproductive system it normally occurs, and which cell results from it.

Explain the difference between embryo and foetus, state at what point in pregnancy the transition from one stage to the other occurs, and mention an important developmental change in each of the three trimesters of gestation.

A hospital analyzes three cases of twin births (birth of two babies in the same delivery):

• Case 1: A boy and a girl with different blood types.

• Case 2: Two girls who share 100% of their DNA.

• Case 3: Two boys who physically look very similar, but genetic testing shows they only share 50% of their genes, just like two siblings born in different years.

1. a) Classify each case as identical twins (monozygotic) or fraternal twins (dizygotic), precisely deducing what occurred during fertilization in each one (number of fertilized eggs, number of sperm, and behavior of the zygote).
2. b) Someone claims: "In Case 1, the boy and the girl must have originated from a single egg that was fertilized simultaneously by two sperm: one with an X chromosome and another with a Y chromosome." Demonstrate why this statement is biologically erroneous and explain how sex was actually determined in that pair of babies.

Put the following stages in order from when the egg cell is fertilised until the baby is born, and write a brief sentence about what happens in each one: foetus, fertilisation, childbirth, zygote, embryo.

Around the middle of pregnancy, an ultrasound scan can already show the baby's sex. Relate this fact to the moment when the organs are formed and differentiated, and explain why an ultrasound scan cannot show it at the beginning of pregnancy.

In a hospital's neonatal unit, two babies are being cared for:

• Case A: A fetus spontaneously expelled at 16 weeks of gestation. It measures about 12 cm and already has a formed heart, kidneys, limbs, and lungs, but is non-viable outside the womb.

• Case B: A baby born at 31 weeks (premature birth) weighing 1,6 kg. It has all its organs, but needs to remain in an incubator for several weeks with constant warmth and assisted breathing to survive.

1. a) Refute the statement of anyone claiming that "since all organs are already formed by week 16, the fetus in Case A should be able to survive on a mechanical ventilator just like the one in Case B." Accurately distinguish between organogenesis (organ formation) and functional maturation.
2. b) Explain which two vital functions were undertaken by the placenta or the mother's body during pregnancy that the baby in Case B is still unable to perform on its own, requiring it to be kept in an incubator.

Look at the image of the foetus inside the uterus. Locate the uterus, the placenta, the umbilical cord and the amniotic fluid, and explain the function of each of these structures in protecting and nourishing the foetus.

Explain what the amniotic sac is and cite two functions the amniotic fluid performs for the foetus.

During pregnancy, the foetus develops inside the uterus attached to the placenta by the umbilical cord. Explain what functions the placenta performs for the foetus and reason why substances consumed by the mother (alcohol, tobacco, certain medications) can affect the baby's development.

In the placenta, the mother's blood and the foetus's blood do not mix, but nutrients, oxygen and waste products pass between them. Explain what type of exchange takes place across the membranes of the placenta and why it is important that the two bloods never mix.

Describe what happens during childbirth: which organ contracts to push the baby out, what path it follows to emerge, and what happens to the umbilical cord after birth.

Childbirth does not happen all at once: three stages are distinguished: the dilation stage of the cervix, the expulsion stage of the baby, and a final stage called the afterbirth (delivery of the placenta). Explain what happens in each of the three stages and what is expelled in the last one.

Explain what a caesarean section is, in which situations it is decided to perform it, and reason why, although today it is a safe operation, it is reserved for cases in which vaginal delivery poses a risk.

During intrauterine life, the fetal lungs are collapsed, filled with fluid, and do not perform gas exchange, as all oxygenation comes from the placenta through the umbilical vein. At the moment of birth, the umbilical cord is cut, and the newborn takes its first cry.

1. a) A relative observes the birth and comments: "We should prevent the baby from crying at birth, because crying exhausts it; it would breathe better in silence and calmly." Refute this statement by justifying the mechanical function that this first forced inspiration, which usually accompanies the cry, performs on the pulmonary alveoli and the fluid they contain.
2. b) Explain what change occurs in the concentration of gases (oxygen and carbon dioxide) in the baby's blood during the first few seconds after clamping the umbilical cord, and deduce how this change stimulates the nervous system to force it to take its first breath.
3. c) Design a flowchart with arrows connecting these four key steps in order: clamping of the umbilical cord / increase in carbon dioxide in the baby's blood / first cry and opening of alveoli / arrival of oxygenated blood via the pulmonary pathway to the entire body.