Pelvis Anatomy Overview

Pelvis and Innominate Bones

• The innominate bones consist of three divisions: Pubis (anterior), Ilum, and Ischium.
• The pubis forms the symphysis pubis, the junction of the innominate bones at the front of the pelvis.
• Sacrum is located above the coccyx and forms the upper part of the pelvic ring.
• Coccyx is comprised of five small, fused bones, allowing some movement during childbirth to accommodate the fetal head.

Obstetric Division of the Pelvis

• False pelvis: The superior half that supports the uterus and aids in fetal descent.
• True pelvis: The inferior half, crucial for childbirth.
• Pelvic inlet: Heart-shaped entrance to the true pelvis where the fetus passes during vaginal birth.
• Pelvic outlet: Bounded by the ischial tuberosities, symphysis pubis, and coccyx, marking the exit for the fetus.
• Pelvic cavity: Curved space between the inlet and outlet that regulates the speed of birth, affects fetal chest compression, and promotes lung fluid expulsion.

Fertilization

• Fertilization, also known as conception or fecundation, is the union of an ovum and spermatozoon, typically occurring in the fallopian tube.
• Only one ovum matures and becomes capable of fertilization for about 24 to 48 hours post-ovulation.
• Sperm can survive in the female body for approximately 48 to 72 hours.
• Successful fertilization depends on sperm maturity, ability to reach the ovum, and penetration capability.

Zygote Formation

• Upon penetration, the sperm and ovum chromosomes fuse to create a zygote with 46 chromosomes (23 pairs).
• A fertilized ovum from an X-carrying sperm results in female offspring (XX); Y-carrying sperm results in male offspring (XY).

Decidua

• After fertilization, the corpus luteum remains active, producing hormones like hCG, necessary for maintaining the uterine lining.
• The decidua is the thickened uterine endometrium during pregnancy, which is categorized into three areas.
• Braxton Hicks contractions occur from about 12 weeks, maintaining pressure in uterine blood spaces.

The Placenta

• A vital organ, weighing 400-600 g at term, serving as a site for maternal-fetal exchange.
• Primarily produces hormones like estrogen, progesterone, and hCG to support pregnancy.
• Estrogen aids mammary gland development; progesterone maintains the endometrial lining and decreases uterine contractions.

Umbilical Cord

• Formed from fetal membranes, the umbilical cord connects the embryo to the placenta, typically measuring about 53 cm long at term.
• Contains one vein (carrying oxygenated blood to the fetus) and two arteries (returning deoxygenated blood to the placenta).
• Wharton’s jelly protects the cord from pressure and prevents vascular obstruction.
• The cord is assessed at delivery for the number of arteries and veins to detect potential anomalies.

Amniotic Fluid

• Produced by the amniotic membrane, the quantity ranges from 800 to 1,200 mL at term, with functions including protection and cushioning for the fetus.
• Inadequate swallowing due to conditions like esophageal atresia can lead to hydramnios (excess fluid) or oligohydramnios (insufficient fluid).
• The fluid is reabsorbed continually, promoting fetal lung development and protein exchange, while offering a protective environment.

Amniotic Membranes

• The outermost layer forms chorionic membranes, providing support to the amniotic sac.
• These membranes have no nerve supply, allowing for painless rupture during labor.

Labor and Delivery

• For successful vaginal delivery, the fetus transitions through the pelvic inlet, cavity, and outlet.
• Proper positioning of the mother during labor ensures optimal placental blood flow and minimizes complications.### Respiratory System Development
• Neural tube differentiates into the central nervous system and neural crest for the peripheral nervous system.
• At 3 weeks, respiratory and digestive tracts begin as a single solid tube that later canalizes.
• By the end of the 4th week, a septum separates the esophagus and trachea; lung buds emerge.
• By 7 weeks, diaphragm development is incomplete; potential for diaphragmatic hernia if it doesn’t close properly.
• Key milestones in respiratory development include:
  • Alveoli and capillaries form between 24-28 weeks; essential for gas exchange.
  • Spontaneous respiratory movements start at 3 months and continue throughout pregnancy.
  • Lung fluid with low viscosity and surface tension forms to assist in alveolar expansion.
  • Surfactant is secreted by alveolar cells around 24 weeks, reducing surface tension and preventing collapse.

Endocrine System Function

• Endocrine organs begin to function when mature, impacting crucial processes:
  • Fetal adrenal glands provide precursors for estrogen synthesis by the placenta.
  • Fetal pancreas produces insulin essential for fetal metabolism.
  • Thyroid and parathyroid glands are vital for metabolic functions and calcium balance.

Digestive System Development

• At 4 weeks, the digestive tract separates from the respiratory tract and starts as a solid structure, hollowing out later.
• By 6 weeks, as the abdomen shrinks, portions of the intestine are pushed into the umbilical cord area until the 10th week.
• Proper rotation of the intestine upon re-entering abdominal cavity prevents complications:
  • Innadequate rotation can cause volvulus, leading to intestinal obstruction.
  • Omphalocele and gastroschisis are congenital anomalies involving intestinal positioning.
• Meconium, a mix of waste materials, begins forming by the 16th week, indicating proper gastrointestinal function.

Nervous System Development

• Nervous system formation occurs actively between the 3rd and 4th weeks.
• High glucose consumption during this phase leaves the mother with mild hypoglycemia, causing symptoms like dizziness.
• Testes develop in the abdominal cavity, with descent into the scrotal sac typically occurring between 34-38 weeks, crucial for male infants’ health.