A 42-year-old woman presents to the emergency department in active labor. She has had no prenatal care and is unsure of the gestational age. Labor progresses rapidly and spontaneous vaginal delivery of a baby boy occurs 3 hours after presentation. On initial exam, the child is 1.9 kg (4.2 lb) with a small head and jaw. A sac-like structure containing intestine, as can be seen in the picture, protrudes from the abdominal wall. What complication is closely associated with this presentation?

Dehydration and necrosis of bowel

Twisting of the bowel around itself

A 31-year-old G3P0 is admitted to the hospital with profuse vaginal bleeding and abdominal pain at 34 weeks gestation. She reports passing bright blood with clots and no water in the discharge. She denies recent trauma or medical illnesses. She had no prenatal care. Her previous pregnancies culminated in spontaneous abortions in the second trimester. She has a 6-year history of drug abuse and cocaine smoking 2 hours prior to the onset of her symptoms. Her blood pressure is 160/90 mm Hg, the heart rate is 93/min, the respiratory rate is 19/min, and the temperature is 36.9℃ (98.4℉). The fetal heart rate is 110/min. On examination, the patient is lethargic. Her pupils are constricted, but reactive to light bilaterally. There are no signs of trauma. Abdominal palpation identifies lower abdominal tenderness and strong uterine contractions. The fundus of the uterus is between the xiphoid process and umbilicus. The patient’s perineum is grossly bloody. On pelvic examination, the vaginal canal is without lesions. The cervix is almost completely effaced and 2 cm dilated. Which of the following options is the most likely cause of the patient’s pregnancy-related condition?

Abrupt constriction of maternal and placental vessels

Thrombosis of the placental vessels

Dramatic decrease in thrombocytes

Premature rupture of the membranes

Rupture of the placental vessels

A 34-year-old G3P2 presents at 33 weeks gestation with several episodes of bloody spotting and mild vaginal bleeding over the past 2 weeks. The bleeding has no specific triggers and resolves spontaneously. She does not report abdominal pain or uterine contractions. She has had two cesarean deliveries. At 20 weeks gestation, the ultrasound examination showed the placental edge to be 5 cm away from the internal cervical os. On examination at this visit, the vital signs are as follows: blood pressure, 110/70 mm Hg; heart rate, 89/min; respiratory rate, 15/min; and temperature, 36.6℃ (97.9℉). The uterus is tender with no palpable contractions and streaks of blood are noted on the perineum, but there is no active bleeding. An ultrasound evaluation shows the placental edge 1 cm from the internal cervical os. Which of the following options best describes the placental position at each ultrasound?

Normally lying placenta → marginal previa

Low-lying placenta → partial previa

Normally placed placenta → low-lying placenta

Marginal previa → partial previa

Low-lying placenta → marginal previa

A 28-year-old male presents to his primary care physician with complaints of intermittent abdominal pain and alternating bouts of constipation and diarrhea. His medical chart is not significant for any past medical problems or prior surgeries. He is not prescribed any current medications. Which of the following questions would be the most useful next question in eliciting further history from this patient?

"Can you tell me more about the symptoms you have been experiencing?"

"Does the diarrhea typically precede the constipation, or vice-versa?"

"Is the diarrhea foul-smelling?"

"Please rate your abdominal pain on a scale of 1-10, with 10 being the worst pain of your life"

"Are the symptoms worse in the morning or at night?"

An 11-year-old girl is brought to the office by her mother due to complaint of intermittent and severe periumbilical pain for 1 day. She does not have any significant past medical history. She provides a history of a recent school trip to the suburbs. On physical examination, there is a mild tenderness around the umbilicus without any distension or discharge. There is no rebound tenderness. Bowel sounds are normal. An abdominal imaging shows enlarged mesenteric lymph nodes, and she is diagnosed with mesenteric lymphadenitis. However, incidentally, a mass of tissue was seen joining the inferior pole of both kidneys as shown in the image. Which of the following best describes this renal anomaly?

Association with ureteropelvic junction obstruction (UPJO)

Fused kidneys ascend beyond superior mesenteric artery.

Rapid progression to acute renal failure

Kidneys are usually non-functional.

Increased risk of developing renal vein thrombosis

Placenta and fetal membranes for USMLE Step 2 CK: High-Yield Anatomy Lessons

Placental Development - The First Hookup

Implantation (Day 6-12): Blastocyst adheres to the uterine wall. The trophoblast rapidly proliferates, differentiating into two key layers.
Trophoblast Layers:
- Cytotrophoblast: Inner, mitotically active cellular layer. Serves as a stem cell source for the syncytiotrophoblast.
- Syncytiotrophoblast: Outer, multinucleated syncytium. Invades the decidua and secretes hCG.
Circulation: The syncytiotrophoblast erodes maternal spiral arteries, creating lacunae that fill with maternal blood, establishing uteroplacental circulation.

⭐ The syncytiotrophoblast lacks MHC-I expression, preventing recognition as foreign tissue by the maternal immune system.

Early placental development and uterine wall invasion

Placental Structure & Circulation - The Fetal Lifeline

Components: Fetal side (Chorionic plate) and Maternal side (Basal plate, decidua basalis). Intervillous space is between them.
Fetal Surface: Smooth, glistening, covered by amnion. Umbilical cord inserts centrally.
Maternal Surface: Rough, lobulated (cotyledons).

Placental Villi Structure and Gas Exchange

Circulation Pathway:

⭐ High-Yield: Maternal and fetal blood do not mix. Gas and nutrient exchange occurs across the placental barrier (syncytiotrophoblast, cytotrophoblast, villus stroma, fetal capillary endothelium).

Placental Functions & Transport - The Border Patrol

Core Functions: Gas exchange (O₂, CO₂), nutrition, waste removal (urea), endocrine synthesis, and immune barrier/transfer.
Transport Mechanisms:
- Simple Diffusion: Gases, water, electrolytes, urea, most drugs.
- Facilitated Diffusion: Glucose (carrier-mediated).
- Active Transport: Amino acids, vitamins, ions (Fe, Ca, I).
- Pinocytosis: Maternal IgG antibodies, providing passive immunity.
Endocrine Factory: Produces hCG, hPL, progesterone, and estrogens.

⭐ Maternal insulin does not cross the placenta, but glucose does. Uncontrolled maternal diabetes can lead to fetal hyperglycemia and subsequent hyperinsulinemia, causing macrosomia.

Facilitated Diffusion of Glucose Across a Cell Membrane

Fetal Membranes & Amniotic Fluid - The Baby Bubble

Fetal membranes and their cellular layers

Amnion & Chorion: Inner (amnion) and outer (chorion) layers enclosing the fetus.
- Amnion: Secretes amniotic fluid; derived from epiblast.
- Chorion: Forms fetal contribution to the placenta.
Amniotic Fluid Functions: Provides cushioning, permits movement/growth, maintains temperature.
Composition & Volume: Initially a maternal plasma ultrafiltrate. From 2nd trimester on, fetal urine is the primary source. Fetal swallowing removes it.

⭐ Oligohydramnios (low fluid, AFI <5 cm) suggests renal anomalies (Potter sequence) or uteroplacental insufficiency. Polyhydramnios (high fluid, AFI >24 cm) suggests impaired swallowing (anencephaly, GI atresia) or high cardiac output.

Placental Abnormalities - When Implantation Wanders

Placenta Accreta Spectrum: Abnormal trophoblast invasion into the myometrium due to a defective decidual layer. A major cause of severe postpartum hemorrhage.
- Accreta: Placental villi attach directly to the myometrium.
- Increta: Villi invade into the myometrium.
- Percreta: Villi perforate through the myometrium, potentially invading adjacent organs (e.g., bladder).
Key Risk Factors: Prior C-section, placenta previa, advanced maternal age.
📌 Mnemonic: Accreta, Increta, Percreta = Adheres, Invades, Perforates.

⭐ Classic presentation involves massive, life-threatening hemorrhage during attempted manual placental separation. Hysterectomy is often the definitive management.

Placenta Accreta, Increta, and Percreta Invasion Depths

High‑Yield Points - ⚡ Biggest Takeaways

The placenta has fetal (chorionic villi) and maternal (decidua basalis) components.

Syncytiotrophoblast secretes hCG to maintain the corpus luteum and lacks MHC-I.

Maternal and fetal blood do not mix; exchange occurs in the intervillous space.

Umbilical cord: two arteries carry deoxygenated blood, one vein carries oxygenated blood.

Polyhydramnios links to swallowing defects; oligohydramnios to renal agenesis (Potter sequence).

Placenta accreta is the abnormal invasion of trophoblasts into the myometrium.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play