Birth

Birth

Birth or parturition is a process whereby a fully developed foetus is expelled out of the mother's womb following the completion of a gestation period. This process goes through three major stages and is greatly controlled by hormones. The first stage is cervical dilation. It starts when true labour begins, that is, when uterine contracts causing the dilation of the cervix up to 10 cm wide. The second stage of parturition is the actual delivery of the baby. It begins when the cervix is dilated completely and it ends with the birth of the baby. The third and the last stage is the expulsion of the placenta and associated membranes out of the womb.

Hormonal control leading to birth

1. Increased sensitivity to oxytocin: As the gestation period nears its end, the uterine wall becomes more sensitive to the hormone oxytocin, which plays a key role in stimulating uterine contractions.
2. Fetal hormonal activity:
   • The hypothalamus of the fetus releases adrenal corticotrophic-releasing hormone (ACTRH).
   • This stimulates the fetal pituitary gland to release adrenal corticotrophic hormone (ACTH).
   • ACTH triggers the fetal adrenal glands to secrete corticosteroids into the mother's blood through the placenta.
3. Effects of corticosteroids on the mother:
   • Decreased Progesterone Levels: Corticosteroids cause a reduction in the production of progesterone, which lowers its levels in the mother's blood.
   • Increased Prostaglandin Secretion: Corticosteroids also stimulate the uterus to produce prostaglandins, which intensify uterine contractions.
4. Labor contractions:
   • The removal of progesterone's inhibitory effect allows the uterine muscles (myometrium) to contract more.
   • The oxytocin hormone promotes these contractions, while prostaglandins increase the power of these contractions, which results in labor pains.

Stages of labor

1. First stage (cervical dilation):
   • The cervix dilates to about 10 cm in diameter, and the hormone relaxin plays a role in this process.
   • The interval between contractions becomes shorter, and contractions become more regular (every 10 to 15 minutes).
   • The mucus plug in the cervix is expelled, and the amniotic sac may rupture, releasing amniotic fluid.
   • This stage lasts between 6 to 12 hours and is characterized by increasing pain and stronger contractions.
   • As contractions continue, the baby's head moves down into the pelvis and causes increased irritation of the cervix, leading to stronger contractions. The first stage ends when the cervix is fully dilated.
2. Second stage (delivery of the baby):
   • The second stage begins when the cervix is fully dilated (10 cm) and ends with the birth of the baby.
   • The baby is pushed through the birth canal, and the umbilical cord is cut after the baby is born to give the baby its independent circulation.
3. Third stage (expulsion of the placenta):
   • This stage occurs 10 to 15 minutes after the delivery of the baby.
   • The placenta and any remaining embryonic membranes are expelled from the uterus.
   • The expulsion of the placenta is crucial to prevent complications, as retained placenta can lead to serious infections or blood poisoning in the mother.

Multiple births

Multiple births occur when more than one baby is born from the same mother and pregnancy. While humans usually give birth to a single child (singleton), multiple births such as twins, triplets, or even quadruplets can occur. Common in mammals like cats, rabbits, dogs, and pigs, multiple births happen when several oocytes are released during ovulation, each fertilized by a separate sperm.

Types of twins

Twins are the most common form of multiple births, and there are two main types:

1. Monozygotic twins (identical twins):
   • These twins originate from the cleavage of the same zygote (fertilized egg).
   • After the initial cleavage, each blastomere (cell resulting from division) develops into an embryo.
   • Monozygotic twins are genetically identical, meaning they share the same DNA and are always of the same sex.
   • They may share the same placenta, but typically, they are enclosed in separate gestational sacs.
2. Dizygotic twins (fraternal twins):
   • These twins are the result of two different egg cells being fertilized by two separate sperm cells.
   • Each twin develops from a different zygote, and thus they are genetically distinct.
   • Dizygotic twins can be of the same or different sex and may resemble each other or other family members.
   • Unlike monozygotic twins, each fraternal twin develops its own placenta and embryonic membranes.

Causes of multiple births

1. Hyperovulation The release of more than one oocyte during ovulation can lead to the fertilization of multiple eggs.
2. Fertility treatments Medications like clomiphene citrate can induce the release of multiple eggs, leading to multiple pregnancies.
3. Genetic factors A twin gene may cause a woman to release multiple eggs during ovulation, increasing the likelihood of multiple births.
4. Maternal age As women age, their ovaries may become more responsive to hormones like FSH, sometimes resulting in multiple ovulations in a single cycle.

This phenomenon can result in twins, triplets, or even larger multiples, though it remains relatively uncommon in humans.