A 22-year-old woman comes to the physician to discuss the prescription of an oral contraceptive. She has no history of major medical illness and takes no medications. She does not smoke cigarettes. She is sexually active with her boyfriend and has been using condoms for contraception. Physical examination shows no abnormalities. She is prescribed combined levonorgestrel and ethinylestradiol tablets. Which of the following is the most important mechanism of action of this drug in the prevention of pregnancy?

Inhibition of rise in luteinizing hormone

Suppression of ovarian folliculogenesis

Prevention of endometrial proliferation

Increase of sex-hormone binding globulin

A 26-year-old gravida 4 para 1 presents to the emergency department with sudden severe abdominal pain and mild vaginal bleeding. Her last menstrual period was 12 weeks ago. She describes her pain as similar to uterine contractions. She has a history of 2 spontaneous abortions in the first trimester. She is not complaining of dizziness or dyspnea. On physical examination, the temperature is 36.9°C (98.4°F), the blood pressure is 120/85 mm Hg, the pulse is 95/min, and the respiratory rate is 17/min. The pelvic examination reveals mild active bleeding and an open cervical os. There are no clots. Transvaginal ultrasound reveals a fetus with no cardiac activity. She is counseled about the findings and the options are discussed. She requests to attempt medical management with mifepristone before progressing to surgical intervention. Which of the following describes the main mechanism of action for mifepristone?

Increase myometrial sensitivity to contractions and induce decidual breakdown

Interferes with cell growth in rapidly dividing cells

Induce teratogenesis in the fetus

Interferes with placental blood supply to the fetus

A 24-year-old woman, gravida 1, at 35 weeks gestation is admitted to the hospital with regular contractions and pelvic pressure for the last 5 hours. Her pregnancy has been uncomplicated and she has attended many prenatal appointments and followed the physician's advice about screening for diseases, laboratory testing, diet, and exercise. She has had no history of fluid leakage or bleeding. At the hospital, her temperature is 37.2°C (99.0°F), blood pressure is 108/60 mm Hg, pulse is 88/min, and respirations are 16/min. Cervical examination shows 60% effacement and 5 cm dilation with intact membranes. Cardiotocography shows a contraction amplitude of 220 MVU in 10 minutes. Which of the following is the most appropriate pharmacotherapy at this time?

No pharmacotherapy at this time

A 23-year-old woman comes to the emergency department because of a 5-day history of nausea and vomiting. There is no associated fever, abdominal pain, constipation, diarrhea, or dysuria. She is sexually active and uses condoms inconsistently. Her last menstrual period was 10 weeks ago. Her temperature is 37°C (98.6°F), pulse is 90/min, respirations are 18/min, and blood pressure is 110/70 mm Hg. There is no rebound tenderness or guarding. A urine pregnancy test is positive. Ultrasonography shows an intrauterine pregnancy consistent in size with an 8-week gestation. The hormone that was measured in this patient's urine to detect the pregnancy is also directly responsible for which of the following processes?

Maintenance of the corpus luteum

Hypertrophy of the uterine myometrium

Stimulation of uterine contractions at term

Thirty minutes after vaginal delivery of a 2780-g (6-lb 2-oz) newborn at term, a 25-year-old woman, gravida 1, para 1, has heavy vaginal bleeding. Her pregnancy was complicated by pre-eclampsia. Her pulse is 111/min and blood pressure is 95/65 mm Hg. Physical examination shows a fundal height 2 inches below the xiphoid process of the sternum. A drug with which of the following mechanisms of action is most appropriate for this patient?

Depolarization of the motor end plate

Increased synthesis of cyclic AMP

Inhibition of norepinephrine reuptake

Binding to prostaglandin I2 receptors

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?"

A 28-year-old woman, gravida 2, para 1, at 14 weeks' gestation, comes to the physician with a 3-day history of abnormal vaginal discharge. She has not had fever, chills, or abdominal pain. One week ago, her 2-year-old daughter had a urinary tract infection that quickly resolved after antibiotic therapy. The patient reports that she is sexually active with one male partner and they do not use condoms. Vital signs are within normal limits. Pelvic examination shows an inflamed and friable cervix. There is mucopurulent, foul-smelling discharge from the cervical os. There is no uterine or cervical motion tenderness. Vaginal pH measurement shows a pH of 3.5. Which of the following is the most appropriate initial step in management?

Nucleic acid amplification test

Potassium hydroxide preparation

A 33-year-old woman, gravida 2, para 1, at 26 weeks' gestation comes to the emergency department because of frequent contractions. The contractions are 40 seconds each, occurring every 2 minutes, and increasing in intensity. Her first child was delivered by lower segment transverse cesarean section because of a nonreassuring fetal heart rate. Her current medications include folic acid and a multivitamin. Her temperature is 36.9°C (98.4°F), heart rate is 88/min, and blood pressure is 126/76 mm Hg. Contractions are felt on the abdomen. There is clear fluid in the vulva and the introitus. The cervix is dilated to 5 cm, 70% effaced, and station of the head is -2. A fetal ultrasound shows polyhydramnios, a median cleft lip, and fused thalami. The corpus callosum, 3rd ventricle, and lateral ventricles are absent. The spine shows no abnormalities and there is a four chamber heart. Which of the following is the most appropriate next step in management?

Perform dilation and evacuation

Physiology of parturition for USMLE Step 3: High-Yield OB/GYN Lessons

Initiation of Parturition - The Starting Pistol

Primary Trigger: Maturation of the fetal hypothalamic-pituitary-adrenal (HPA) axis.
Key Hormonal Shift: Functional progesterone withdrawal results in an increased Estrogen:Progesterone ratio, removing the "progesterone block" on uterine contractility.

Estrogen's Pro-Labor Effects:
- Upregulates oxytocin receptors in the myometrium.
- Increases prostaglandins ($PGF_{2\alpha}$, $PGE_2$) causing cervical ripening & contractions.
- Forms gap junctions (connexin-43) for coordinated contractions.

⭐ Fetal cortisol not only initiates labor but is also crucial for fetal lung maturation by stimulating surfactant production.

Hormonal Control - The Chemical Conductors

A pivotal shift from uterine quiescence to activation occurs as the estrogen-to-progesterone ratio ↑, favoring estrogen. This change initiates the entire parturition cascade.

Hormone	Source	Primary Role in Labor
Estrogen (Estradiol)	Placenta	↑ Oxytocin receptors, ↑ myometrial gap junctions, promotes cervical softening.
Progesterone	Placenta	Maintains uterine quiescence ("pro-gestation"); its functional withdrawal is key.
Oxytocin	Post. Pituitary	Potent uterotonic; stimulates forceful, rhythmic uterine contractions.
Prostaglandins (PGE₂, PGF₂α)	Decidua, Membranes	Induce cervical ripening and stimulate myometrial contractility.

Myometrial Activity - The Engine of Labor

Uterine Contractions: Involuntary, rhythmic, and forceful contractions originating from pacemaker cells near the uterine fundus.
- Coordination: Propagate downwards (fundal dominance), ensuring efficient fetal expulsion.
- Gap Junctions: ↑ expression of connexin-43 forms gap junctions between myometrial cells, allowing synchronized, coordinated contractions.
- Intracellular Action: ↑ intracellular $Ca^{2+}$ activates myosin light-chain kinase (MLCK), triggering the actin-myosin interaction.
Ferguson Reflex (Positive Feedback Loop):

Result of Contractions:
- Cervical Effacement: Thinning and shortening of the cervix.
- Cervical Dilation: Opening of the cervical os, measured from 0 to 10 cm.

Physiology of uterine contractions showing fundal dominance

Cervical Changes - Ripening & Dilation

Cervical effacement and dilation stages

Cervical Ripening (Softening): A prerequisite for labor where the cervix becomes soft and compliant.
- Mechanism: Breakdown of collagen fibers, ↑ hyaluronic acid, and ↑ water content.
- Mediators: Prostaglandins (PGE₂, PGF₂α), nitric oxide, and relaxin.
Effacement: The progressive thinning and shortening of the cervix, measured from 0% to 100%.
Dilation: The opening of the cervical os, measured from a closed state (0 cm) to full dilation (10 cm).

⭐ The Bishop Score is used to assess cervical favorability for labor induction; a score ≥ 8 is considered favorable.

High-Yield Points - ⚡ Biggest Takeaways

Labor begins via functional progesterone withdrawal and estrogen dominance, increasing oxytocin receptors and gap junctions.

Prostaglandins (PGE2, PGF2α) are critical for cervical ripening and stimulating powerful myometrial contractions.

The Ferguson reflex is a positive feedback loop where cervical stretch triggers oxytocin release, intensifying contractions.

Fetal cortisol from the maturing HPA axis is a key trigger for initiating parturition.

Connexin-43 forms gap junctions, allowing myometrial cells to contract synchronously for effective labor.

Continue reading on Oncourse

CONTINUE READING — FREE

or get the app

App Store|Google Play