Which of the following conditions must be met for fertilization to occur? Select all that apply

Fetal growth restriction (FGR) is a condition where the fetus fails to achieve its genetically predetermined growth potential, often due to placental insufficiency, leading to chronic hypoxia and nutrient deprivation. Oligohydramnios, defined as an amniotic fluid index less than 5 cm or a single deepest pocket less than 2 cm, frequently accompanies FGR. Amniotic fluid volume depends primarily on fetal urine production, lung fluid secretion, and swallowing. Placental dysfunction, fetal renal anomalies, and altered fetal circulation can decrease urine output, reducing amniotic fluid and causing oligohydramnios.

Rationale for correct answers

A. Reduced fetal urine output occurs secondary to placental insufficiency, which causes hypoxia and redistribution of blood flow, limiting kidney perfusion and thus decreasing urine production—the major source of amniotic fluid in the third trimester.

C. Congenital anomalies of the fetal renal system impair urine formation or excretion, directly reducing amniotic fluid volume, as fetal urine constitutes up to 90% of amniotic fluid after 16 weeks gestation.

E. A shift in fetal blood flow away from the kidneys (known as blood flow redistribution or “brain-sparing”) prioritizes vital organs during hypoxia, reducing renal perfusion and urine output, thus contributing to oligohydramnios.

Rationale for incorrect answers

B. Increased fetal swallowing of amniotic fluid is not a typical compensatory mechanism in FGR; rather, swallowing usually maintains normal fluid balance and is not significantly increased in FGR or oligohydramnios.

D. Absence of functional kidneys (e.g., bilateral renal agenesis) leads to anuria and severe oligohydramnios, but this is a specific congenital anomaly rather than a common cause of FGR-related oligohydramnios.

Take home points

• Placental insufficiency decreases fetal renal perfusion, reducing urine output and amniotic fluid.
• Fetal renal congenital anomalies can impair urine production, leading to oligohydramnios.
• Fetal blood flow redistribution prioritizes brain over kidneys, reducing renal function.
• Oligohydramnios is a frequent finding in FGR and indicates compromised fetal well-being.

Fertilization is the union of a mature sperm and ovum resulting in a zygote. Successful fertilization depends on sperm capacitation, which is the biochemical modification of sperm in the female reproductive tract that enables penetration of the ovum. Sperm must exhibit adequate motility to traverse the cervical mucus, uterus, and fallopian tubes. The cervical mucus undergoes cyclical changes, becoming thin and less viscous during ovulation to facilitate sperm passage. The ovum’s viability is typically 12 to 24 hours post-ovulation, not 48 hours, and sperm are usually deposited in the vagina, not directly in the uterus.

Rationale for correct answers

B. Capacitation involves the removal of glycoproteins and seminal plasma proteins from sperm, which enables acrosomal reaction necessary for ovum penetration.

D. Sperm motility is essential for migration through the cervix, uterus, and fallopian tubes, overcoming the challenges of the female reproductive environment.

E. Thin, less viscous cervical mucus during ovulation reduces resistance, allowing sperm to swim effectively toward the ovum.

Rationale for incorrect answers

A. Sperm are normally deposited in the vagina during coitus, not directly in the uterus; uterine deposition is not a prerequisite for fertilization.

C. The ovum remains viable for approximately 12 to 24 hours after ovulation; viability for 48 hours is incorrect, as the fertilization window is shorter.

Take home points

• Capacitation is necessary to activate sperm for fertilization.
• Adequate sperm motility is crucial for successful sperm migration.
• Cervical mucus consistency changes to facilitate sperm passage during ovulation.
• Ovum viability after ovulation is limited to about 12 to 24 hours.