Obstetrics and Pregnancy Key Concepts

Gestational Length

• Gestational length is approximately 280 days, plus or minus 10 days, or 40 weeks from the last menstrual period (LMP) with a margin of error of 10 days.
• Conception typically occurs 14 days after the LMP which makes the gestational length 266 days (+/- 10 days).

Gestational Age vs. Embryonic Age

• Gestational age= 40 weeks from LMP
• Embryonic age= 38 weeks from conception.
• Obstetric clinical management follows gestational age from the LMP, not embryonic age.

Ordinal vs. Cardinal Reference

• Cardinal references are exact (e.g., 12 weeks 2 days).
• Ordinal references add one week to the cardinal week (e.g., 12 weeks 2 days = 13 weeks).

Developmental Stages: Trimesters

• Zygote stage: 1-4 weeks, first trimester (0-13 weeks).
• Embryo stage: 5-10 weeks, second trimester (14-27 weeks).
• Fetal stage: 11-40 weeks, third trimester (28-40 weeks).

Terminology

• Gravida (Gx): The total number of pregnancies, including the current one (e.g., G2).
• Para (GxPxxxx): The number of pregnancies carried to term (e.g., GxP1234).
  • The '1' represents term pregnancies (37+ weeks).
  • The '2' represents premature deliveries (20-36 weeks).
  • The '3' represents early loss or early demise (therapeutic/elective or ectopic).
  • The '4' represents delivered living/living children.
• Nullipara: A woman during her first pregnancy.
• Multipara: A woman who has had 2-4 pregnancies.
• Grand multipara: A woman who has had 5 or more successful pregnancies.

Biomedical Ethical Considerations in Obstetrics

• Non-maleficence: "Do no harm"; balance risks with benefits.
  • The bioeffects of ultrasound are not fully known.
  • As Low As Reasonably Achievable (ALARA) must be practiced.
• Beneficence: Procedures should benefit the patient.
  • Benefit should outweigh harm if ALARA is followed.
• Autonomy: Respect the mother's values, beliefs, and choices, as the fetus does not have autonomy by law.
• Justice: Ensure equal access to quality examiners and procedures.
  • Timing of procedures affects treatment and diagnosis.
• Confidentiality: Follow HIPAA regulations.
• Veracity: Truthfulness.
• Fidelity: Maintain integrity and competence within the scope of practice.

Practice ALARA – Bioeffect Parameters

• Mechanical Index (MI): Measures potential cell damage from gas-filled bubbles.
  • MI should not exceed 1.0 in OB imaging.
• Thermal Index (TI): Measures potential cell damage from tissue heating.
  • TIS (soft tissue) for first trimester imaging (up to 10 weeks).
  • TIB (bone) after 10 weeks when bones begin to mineralize.
  • Reduce TI if TIS exceeds 0.7 or TIB exceeds 0.5, and limit scan time.
• Parameters affecting MI and TI:
  • High transducer frequency.
  • Long dwell time.
  • Doppler imaging transmits the most power.
    • M-mode suggested for heartbeat unless otherwise indicated.
  • Avoid scanning mothers with fevers.

Reproduction Overview: Gamete Formation

• Male and female reproductive cells are called gametes.
• Each gamete (spermatozoa & oocyte) are haploid cells.
  • Haploid cells have 23 chromosomes.
• Fertilization combines two haploid cells to form a diploid zygote.
  • Diploid cells have 46 chromosomes.

Ovulation

• Follicles on the ovary contain immature oocytes.
• The hypothalamus releases gonadotropin which stimulates the pituitary gland to release FSH and LH.
• FSH stimulates one oocyte to mature and secrete estrogen.
• Estrogen stimulates the endometrium to thicken and triggers a spike in LH.
• LH stimulates ovulation, the release of the mature oocyte.
• The ruptured follicle becomes a corpus luteum, producing progesterone.
  • Remaining ovarian follicles undergo atrophy.
• If fertilization occurs:
  • The corpus luteum continues to produce progesterone and estrogen.
  • The endometrium thickens and becomes 'decidualized'.
  • Fertilization typically occurs in the ampulla of the fallopian tube.
  • Trophoblast (outer cells of the zygote) secretes hCG to sustain the corpus luteum until 12 weeks.
• If fertilization does not occur:
  • Estrogen and progesterone levels drop, leading to menstruation.

Zygote Stage: Fertilization and Implantation

• Fertilization occurs in the ampulla of the fallopian tube.
• The fertilized ovum (zygote) divides to form a morula.
• The morula reorganizes into a blastocyst that has an organized, hollow form..
  • Enters the uterus and is nourished by the decidua (thickened endometrium).
  • Inner cell mass (embryoblast) forms the embryo.
  • Outer cells (trophoblast) form the placenta and membranes.
  • Fluid-filled cavity between cell masses is the blastocele.
  • Implantation occurs ~7 days after fertilization.
• Trophoblast layers:
  • Inner cytotrophoblast forms the chorion, amnion, and connecting stalk.
  • Outer syncytiotrophoblast invades the decidua for implantation.
    • Forms lacunae that develop into intervillous spaces and the placenta.
    • Secretes hCG to sustain the corpus luteum.
• Decidual Reaction of the Endometrium:
  • Progesterone transforms endometrial cells into glycogen and lipoid cells.
  • Making it 'decidualized' which allows implantation to occur.
  • Decidua layers:
    • Decidua basalis: Implantation site attaches to the chorion frondosum (placenta).
    • Decidua capsularis: Closes over the blastocyst and becomes smooth.
    • Decidua parietalis (vera): Covers the remaining endometrial cavity.

Early Development of Placenta

• Chorionic Membrane (Chorionic Sac):
  • Outer membrane formed from trophoblasts.
  • Layers:
    • Chorion frondosum: Forms chorionic villi, the site of implantation, and develops into the placenta.
    • Chorion leave: Smooth chorion present everywhere except at the implantation site.
• Extraembryonic Coelom (Chorionic Cavity):
  • The cavity between the amnion and chorion.
  • The YS is located within.
• Amniotic Membrane (Amniotic sac):
  • Inner membrane.
  • Surrounds epiblast cells; the fetus develops within.
  • Covers the umbilical cord from placenta to the fetal umbilicus.
• Amnion and Chorion:
  • Membranes fuse into one by 15 weeks.
• Yolk Sac (YS):
  • Primary YS disappears quickly.
  • Secondary YS (umbilical vesicle):
    • First structure seen within the gestational sac (GS) via sonography.
    • Connects to the fetal midgut via the vitelline duct (yolk stalk).
    • Located in the extraembryonic coelom outside the amnion.
    • Functions: provides nutrients and hematopoiesis.
    • Normal size: 3mm.

Week 5

• Neural plate and folds of the fetus are present but the fetus may not be visualized sonographically yet.
• Yolk Sac (YS) is seen
  • Round with smooth walls
  • Located between amniotic and chorionic membranes
  • YS should be seen when the mean GS diameter(MSD) is 8mm or more or when hCG is ≥3000 mIU/ml
  • NL YS size is < 6mm in diameter
  • Double Bleb sign is the YS with amniotic membrane seen together as two bubbles

Week 6

• Embryo is seen adjacent to the YS (should be seen by 6wk or 2.5cm MSD)
• Heart contraction begins with unidirectional blood flow
  • Embryo heart beats by 7mm CRL
  • Normal 1st trimester HR 90-175 bpm (beats per minute)

Week 7

• Brain has a single vesicle
• Embryo is C-shaped
• The heart bulges from the body

Week 8

• Limb buds seen
• Brain has 3 vesicles
• Midgut herniates into the umbilical cord (normal to herniate between 8-12weeks)

Key obstetrics concepts, including gestational age calculation, pregnancy dating conventions (cardinal and ordinal), and understanding gravidity and parity notation (G3P1102). Also covers fetal development stages, ethical principles like Non-maleficence, trimesters, and grand multiparity.