Introduction to Pregnancy:
Sexual reproduction is the process by which organisms produce offspring through the formation of gametes. Male gametes are called sperm (spermatozoa) and female gametes are called secondary oocytes. The organs that produce gametes are known as gonads — the testes in males and the ovaries in females.
Once sperm have been deposited in the female reproductive tract and a secondary oocyte has been released from the ovary, fertilization can occur. This process initiates a cascade of developmental events that ultimately results in the birth of a healthy baby.
Pregnancy is a sequence of events that begins with fertilization, proceeds through implantation, embryonic development, and fetal development, and typically ends with childbirth after about 38 weeks (or approximately 40 weeks after the last menstrual period).
Developmental Biology:
Developmental biology studies the sequence of events from fertilization to the formation of an adult organism. From fertilization through the eighth week, the developing human is called an embryo, and this period is known as the embryonic period. The study of this phase is termed embryology.
The fetal period begins at week nine and continues until birth, during which the developing human is called a fetus.
The Placenta and Its Functions:
During pregnancy, a vital structure called the placenta forms between the mother and the developing baby. The placenta, formed by the chorion and uterine tissue, plays several crucial roles:
- Provides nutrients and oxygen to the growing fetus.
- Removes waste materials from the fetus, functioning like the digestive and excretory systems.
- Releases hormones such as human placental lactogen, progesterone, estrogen, human chorionic gonadotropin (hCG), and relaxin essential for fetal growth.
- The umbilical cord connects the fetus to the placenta, transporting nutrients and oxygen from the mother to the fetus.
Trimesters of Pregnancy:
Prenatal development — the time from fertilization to birth — includes both embryonic and fetal periods. It is divided into three trimesters:
- First Trimester: The most critical stage, when major organ systems begin to form. The developing embryo is highly sensitive to drugs, radiation, and infections.
- Second Trimester: Organ systems near completion, and the fetus assumes distinct human features.
- Third Trimester: Characterized by rapid fetal growth and doubling of weight. Most organ systems become fully functional.
Parturition (Childbirth):
Parturition refers to childbirth or labor — the process of delivering the baby after the pregnancy period. The onset of labor is triggered by hormonal signals, particularly cortisol.
During labor, the cervix dilates and relaxes. Hormones such as oxytocin, estrogen, and cortisol are released, which stimulate uterine contractions and prepare for milk production.
The uterus contracts to push the fetus toward the cervix and through the birth canal, with the baby’s head emerging first. After childbirth, the uterus expels the placenta, and lactation begins. The first milk, known as colostrum, is rich in antibodies that help protect the newborn against infections.
Signs and Symptoms of Parturition:
- Bloated abdomen
- Mucous discharge
- Restlessness and irritability
- Labour pains and contractions
- Swollen vulva and relaxed pelvic ligaments
- Development of mammary glands with milk secretion
Stages of Parturition:
1) Dilation Stage:
This stage begins with the onset of labor and continues until the cervix is fully dilated. It has two phases:
- Latent Phase: Cervical dilation from 0–4 cm. It takes around 6 hours for first-time mothers and about 5 hours for subsequent deliveries.
- Active Phase: Cervical dilation from 4–10 cm, occurring at a rate of about 1 cm per hour in first-time mothers and 2 cm per hour in subsequent deliveries.
2) Expulsion Stage:
This stage starts after full dilation and continues until the baby is born. It includes:
- Passive Phase: The baby’s head begins to move out through the vagina.
- Active Phase: The mother pushes using abdominal muscles along with uterine contractions, leading to childbirth. The umbilical cord is clamped after delivery.
3) Placental Stage:
This stage involves the separation and expulsion of the placenta. Uterine contractions decrease simultaneously after the placenta is delivered.
Hormones Involved in Parturition
- Estrogen: Plays a key role in stimulating uterine contractions.
- Oxytocin: Facilitates uterine contractions during labor and aids in milk ejection during breastfeeding.
- Relaxin: Relaxes pelvic ligaments and widens the cervix for delivery.
- Prostaglandin: Synthesized within the amnion and chorion; it helps in cervical ripening, membrane changes, and myometrial contractions.
Detailed Notes:
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