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Physiology and Diagnosis of Pregnancy

Physiology of Pregnancy

• Pregnancy: The process of embryo growth within the mother's body.
• Start of Pregnancy: Fertilization of a mature ovum.
• End of Pregnancy: Expulsion of the fetus and its associated structures from the mother's body.

Fertilization

• Definition: A capacitated sperm fuses with a secondary oocyte, occurring in the ampullary-isthmic junction of the fallopian tube: fertilization.
• Capacitation of Sperm: Occurs in the fallopian tubes.
• Acrosome Reaction:
  • Occurs after capacitation.
  • The acrosomal membrane of the capacitated sperm ruptures, releasing enzymes that dissolve the corona radiata and zona pellucida surrounding the oocyte.
• Zona Pellucida Reaction:
  • Occurs after the acrosome reaction.
  • Prevents other sperm from entering the oocyte.
  • Physiologically, double fertilization is not possible under normal circumstances.

Implantation

• Definition: Approximately 6-7 days (1 week) post-fertilization, the embryo implants into the uterine lining, with a short window of opportunity.
• Process of Implantation: Localization, adhesion, invasion.

Leukocytes: Recognition, adherence, degradation.

• Conditions:
  • Disappearance of the zona pellucida.
  • Syncytiotrophoblast (from the trophoblast of the blastocyst).
  • Synchronization between the blastocyst and the uterine lining.
  • Adequate levels of estrogen and progesterone in the pregnant woman.

• Capacitation Site: Uterine cavity and fallopian tubes.
• Site of Fertilization: Ampullary-isthmic junction of the fallopian tube.
• Ectopic Pregnancy Common Site: Ampulla.
• Endometriosis Common Site: Ovary > uterosacral ligament.

Stages of Fetal Development

• Pregnancy Weeks: Calculated from the first day of the last menstrual period, hence 2 weeks need to be subtracted to determine the week of conception.
• Calculation of Gestational Age: From the first day of the last menstrual period.
• Embryo: Up to the end of the 10th week of gestation (10 weeks of pregnancy), or the end of the 8th week of fetal age. Differentiation and formation.
• Fetus: From the 11th week of gestation (11 weeks of pregnancy) or from the 9th week of fetal age. Growth and maturation.
• Timeline (all ages are fetal ages!):
  • Embryo: From fertilization to the 8th week of fetal age.
  • Fetus: From the 8th week of fetal age to birth.
    • Preterm Infant: 28 - 37 weeks.
    • Term Infant: 37 - 42 weeks.
    • Post-term Infant: Greater than 42 weeks.
  • Infant: From birth to 1 year.
    • Newborn: From birth to 4 weeks post-birth.
  • Toddler: 1 year to 3 years.
  • Preschool Age: 3 years to 6 years.
  • School Age: 6 years to 12 years.
  • Adolescence: 12 years to 18 years.

Developmental Characteristics

• 4 weeks (28 days, one menstrual month) is the unit of gestational age, totaling 280 days or 40 weeks.
• 8 weeks: Heart forms, fetal movement visible on ultrasound.
• 12 weeks: Gender can be recognized.
• First Felt Fetal Movements: Some at 16 weeks, generally at 20 weeks.
• Heart Sounds Heard: End of 20 weeks.
• "First Five Square, Last Five Multiply"
  • First five months of fetal length: ( L = m^2 ).
  • Last five months of fetal length: ( L = m \times 5 ).

Anatomy

Vascular Structure	Number	Clinical Significance
Umbilical Arteries	2	Close postnatally as the round ligament of the liver. Assess fetal blood supply and placental function; changes in the S/D ratio may indicate intrauterine growth restriction or placental dysfunction.
Umbilical Vein	1	Closes postnatally as the ligamentum teres.
Ductus Arteriosus	1 (open prenatally, closes postnatally)	Allows blood flow from the pulmonary artery to the aorta during fetal life. Closes 2-3 months after birth. Abnormalities may lead to cardiac issues.
Foramen Ovale	1 (closes in most cases postnatally)	Allows blood flow between the right and left atria during pregnancy. Closes mostly by 6 weeks postnatally. Persistence may indicate congenital heart disease.

• Blood:
  • Hematopoiesis begins in the yolk sac at 3 weeks of fetal age, followed by the liver, bone marrow, and spleen (Ref: Pediatrics).
  • Granulocytes appear in fetal blood at 8 weeks of gestation, lymphocytes in the thymus and spleen at 12 weeks.
• Respiration:
  • Thoracic wall movements visible on ultrasound at 11 weeks, breathing movements in amniotic fluid after 16 weeks.
• Digestion:
  • Intestinal peristalsis begins at 11 weeks, basic digestive system established by 16 weeks.
• Urogenital:
  • Urination occurs by 11-14 weeks, bladder contains urine by 14 weeks.
• Endocrine:
  • Thyroid and pancreas begin developing at 6 weeks, "Double" by 12 weeks: thyroid hormones and insulin.
• Reproductive:
  • Male: Testes: 9 weeks; Female: Ovaries: 11-12 weeks.