Early Dating Scan is the Best Approach
The most effective intervention to prevent postterm pregnancy in a woman with a prior postterm delivery is an early dating ultrasound scan, ideally performed in the first trimester before 14 weeks of gestation. This is the only evidence-based strategy that directly addresses the prevention of postterm pregnancy diagnosis and its associated complications.
Why Early Dating Scan (Option A) is Correct
Accurate gestational age determination through first-trimester ultrasound significantly reduces the incidence of postterm pregnancy diagnoses by eliminating dating errors based on last menstrual period alone 1, 2.
The use of ultrasound for early pregnancy dating substantially decreases the number of pregnancies incorrectly classified as postterm compared to menstrual dating, which is the most common reason for postterm pregnancy diagnosis 3, 4.
Early ultrasound dating is associated with reduced need for labor induction by preventing false postterm diagnoses 5.
A history of prior postterm pregnancy increases the risk of recurrence, making accurate dating even more critical in this population 3.
Why the Other Options Are Incorrect
Progesterone at 16 Weeks (Option B)
Progesterone is indicated for prevention of preterm birth, not postterm pregnancy 6.
17-alpha hydroxyprogesterone caproate (17P) is recommended for women with prior spontaneous preterm birth between 20-36 weeks, starting at 16-20 weeks 6.
There is no evidence that progesterone prevents postterm pregnancy or has any role in women with a history of postterm delivery 6.
Aspirin (Option C)
Aspirin (100-150 mg daily from week 12) is indicated for prevention of pre-eclampsia in high-risk women, not for prevention of postterm pregnancy 6.
This patient's history of postterm delivery at 41 weeks does not constitute a high-risk indication for aspirin prophylaxis 6.
Ambulation in Late Pregnancy (Option D)
There is no evidence that encouraging ambulation prevents postterm pregnancy 1, 3.
While membrane stripping/sweeping may help prevent postterm pregnancy, routine ambulation has not been validated as an effective intervention 3.
Management Algorithm for This Patient
Perform first-trimester dating ultrasound between 6-14 weeks to establish accurate gestational age 1, 2.
Counsel the patient at term about induction timing: The American College of Obstetricians and Gynecologists recommends considering elective induction at 39 weeks in low-risk nulliparous patients and recommends induction by 42 weeks in all patients 1, 2.
If expectant management is chosen beyond 41 weeks, initiate antepartum fetal surveillance starting at 41 weeks' gestation to mitigate risks of perinatal morbidity and mortality 1, 2.
Discuss that induction at 41 weeks reduces perinatal mortality and stillbirth compared to expectant management or waiting until 42 weeks 1, 5.
Key Clinical Pitfall
The critical error would be confusing preterm birth prevention strategies (progesterone) with postterm pregnancy management. These are opposite ends of the gestational age spectrum requiring completely different interventions 6, 1.