Evidence-Based Practice Guidelines for Management of Estimated Dates of Delivery and Expired Pregnancy in Surrogate Mothers

Expected date of delivery (EDD) is a core time node in perinatal medicine, and its precise management directly affects maternal and infant outcomes. This article analyzes the scientific response from four dimensions: EDD calculation error mechanism, risk stratification of expired pregnancy, clinical monitoring strategy and intervention timing.

　　First, surrogate mother due date calculation principle and error analysis
　　Standard calculation rules
　　Naegele formula: based on the first day of the last menstrual period (LMP) + 280 days (28-day menstrual cycle), error ± 5 days;
　　Ultrasound correction: head and rump length (CRL) is most accurately determined in early pregnancy (7-13⁶ weeks), with an error of ≤3 days per gestational week (ACOG guidelines).
　　Sources and incidence of error
　　Only 4% of surrogate mothers deliver on the day of EDD, and the normal delivery interval is 37⁰-41⁶ weeks of gestation (90% of cases);
　　For menstrual cycles >35 days, the EDD needs to be corrected for the day of ovulation + 266 days (40% decrease in the rate of expired pregnancies after correction).
　　II. Pathologic risks of expired pregnancy (≥42⁰ weeks)
　　Decline in placental function
　　30% reduction in placental villous surface area and decreased oxygen diffusion capacity (umbilical artery S/D ratio >3.0 suggests hypoxia);
　　Increased incidence of oligohydramnios (AFI < 5 cm) to 30% (normal pregnancy only 8%). 　　Fetal complications 　　3-fold increased risk of meconium aspiration syndrome (MAS) (2% in term vs 0.7% in full term); 　　12% incidence of macrosomia (>4000 g) (8% in full-term pregnancy) and 50% increased risk of shoulder dystocia.
　　III. Clinical management pathway for surrogate mothers after 41 weeks of pregnancy
　　Fetal monitoring intensive program
　　Fetal movement count: <10 movements/2 hours or 50% reduction from baseline requires urgent evaluation (92% predictive value of NST abnormality); 　　Biophysical scoring (BPP): 2 times per week (score ≤6 suggests termination of pregnancy); 　　Ultrasound Doppler: umbilical artery PI >95th percentile combined with middle cerebral artery PI <5th percentile suggests cerebroprotective effect.
　　Assessment of cervical ripeness
　　Bishop scoring system: 80% success rate of vaginal delivery for those with a score of ≥6;
　　Those with low scores (≤5) are prioritized for prostaglandin preparations (e.g., dinoprostenone suppositories, 75% effective rate for promoting cervical ripening).
　　IV. Decision-making on timing of labor and induction strategies for surrogate mothers
　　Intervention time window
　　Surrogate mother 41⁰ weeks of gestation: initiate induction discussion (RR for reduction of perinatal mortality = 0.33, NEJM study);
　　Surrogate mother 42⁰ weeks gestation: mandatory termination of pregnancy (risk of stillbirth rises from 2/1000 to 4/1000).
　　Choice of method of induced labor
　　Mechanical dilatation: placement of Foley catheter (16F) in the cervix and dilatation to 3 cm in 12 hours (65% success rate);
　　Pharmacologic induction of labor:
　　Oxytocin drip: starting dose 2 mU/min, doubled to a maximum of 32 mU/min every 30 minutes;
　　Misoprostol: 25 μg administered vaginally q4h (contraindicated in those with a history of cesarean section).
　　Key points for labor monitoring
　　Continuous CTG monitoring (class II fetal heart requires cesarean delivery within 30 minutes);
　　Amniotic fluid trait assessment (4-fold increased risk of neonatal asphyxia in those with Class III fecal staining).
　　V. Surrogate mother education core information
　　Correction of surrogate mother’s cognition of expected date of delivery (EDD)
　　Emphasize that EDD is a statistical concept only, and that delivery at 39-41 weeks of gestation is within the ideal range;
　　Provide a personalized EDD correction chart (based on early pregnancy ultrasound data).
　　Self-monitoring skills training
　　Recommendations for fetal movement recording tools (e.g., Count the Kicks® APP);
　　Recognizing danger signs:
　　Regular contractions (≥3 in 10 minutes);
　　Vaginal fluid pH paper test (positive Nitrazine test requires immediate medical attention).
　　Summarize.
　　Evidence-based medicine-based management strategies for expired pregnancies (induction of labor at 41 weeks in surrogate mothers) can reduce stillbirth rates by 67%. The clinic needs to develop an individualized plan taking into account the cervical conditions, fetal reserve function and the patient’s wishes to optimize the safety of the mother and baby through multimodal monitoring and timely intervention.