1.2 Prenatal History, Obstetric Notation & Gestational Dating

The initial prenatal evaluation is the cornerstone of antepartum midwifery care. It establishes baseline maternal physiological parameters, identifies preexisting medical or genetic risks, accurately determines the estimated date of delivery (EDD), and initiates a partnership grounded in shared decision-making. For the AMCB examination, the certified nurse-midwife (CNM) must demonstrate mastery of obstetric terminology, accurate gestational dating rules, baseline laboratory interpretation, and counseling regarding genetic screening modalities.

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Comprehensive Health History & Risk Stratification

The first prenatal encounter requires a systematic health history encompassing medical, surgical, gynecologic, genetic, psychiatric, and social domains.

Universal Screenings in Early Pregnancy

• Intimate Partner Violence (IPV): ACOG and ACNM mandate universal, private, face-to-face screening at the initial visit, repeated once per trimester and postpartum. Screening must occur when the patient is unaccompanied. Validated screening tools (e.g., HARK, HITS) should be used, followed by trauma-informed safety planning and resource provision when disclosures occur.
• Substance Use: Universal screening using validated verbal instruments (such as the 4Ps Plus, CRAFFT for adolescents, or 5Ps) under the Screening, Brief Intervention, and Referral to Treatment (SBIRT) framework. Urine drug toxicology testing should never be ordered without explicit, informed patient consent.
• Depression & Anxiety: Baseline screening using validated questionnaires (Edinburgh Postnatal Depression Scale [EPDS] or Patient Health Questionnaire-9 [PHQ-9]) to identify preexisting or emerging perinatal mood disorders.

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Obstetric History Documentation: The GP (TPAL) System

Precise obstetric notation provides immediate insight into a patient's reproductive history and recurrence risks for complications such as preterm birth or recurrent pregnancy loss.

Definitions

• Gravida (G): Total number of pregnancies, regardless of duration, location (intrauterine vs. ectopic), or clinical outcome. The current pregnancy is included in the count.
• Para (P): Number of completed pregnancies that have reached the threshold of viability (traditionally defined as ≥20 0/7 weeks of gestation or a fetal birth weight of ≥500 grams), regardless of whether the infant was born alive or stillborn.

The TPAL Four-Digit Parity Breakdown

To communicate obstetric history with clinical precision, the TPAL system is standard:

1. T (Term): Deliveries occurring at ≥37 0/7 weeks of gestation (early term, full term, late term, or postterm).
2. P (Preterm): Deliveries occurring between 20 0/7 and 36 6/7 weeks of gestation.
3. A (Abortion): Pregnancies ending prior to 20 0/7 weeks of gestation (includes spontaneous pregnancy losses, induced abortions, ectopic pregnancies, and hydatidiform moles).
4. L (Living): Number of biological children currently alive.

[!IMPORTANT] On the AMCB exam, pay close attention to twins and higher-order multiples. A twin delivery at 35 weeks counts as one preterm delivery (P = 1), but yields two living children (L = 2). A woman who delivers twins at 38 weeks is G1 P1 (TPAL: 1-0-0-2).

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Estimated Date of Delivery (EDD) Determination

Accurate gestational age assignment directs all subsequent clinical care, including the timing of aneuploidy screening, anatomy ultrasound, surveillance for fetal growth disorders, and postterm management.

Menstrual Dating: Naegele's Rule

Naegele's rule calculates the EDD based on a standard 28-day menstrual cycle where ovulation occurs precisely on cycle day 14: $\text{EDD} = \text{First day of Last Normal Menstrual Period (LMP)} + 7 \text{ days} - 3 \text{ months} + 1 \text{ year}$

Example: An LMP starting on May 12, 2026 yields an EDD of February 19, 2027 (May 12 + 7 days = May 19; May - 3 months = February).

Adjustments for Non-28-Day Cycles: If a patient has regular cycles of a different length, calculate: $\text{Adjustment} = \text{Cycle Length} - 28 \text{ days}$ Add or subtract these days accordingly. For a regular 35-day cycle, add 7 additional days to the calculated EDD.

Ultrasound Dating Discrepancy Criteria

Menstrual dating assumes regular ovulatory cycles and precise patient recall. When an early ultrasound measurement disagrees with the LMP dating, the midwife must apply the ACOG / SMFM / AIUM consensus criteria to decide whether to retain the LMP date or redate to the ultrasound.

Crown-rump length (CRL) measured in the first trimester is the most accurate fetal biometric parameter, with a margin of error of ±5 to 7 days.

Special Case (Assisted Reproductive Technology): For pregnancies conceived via in vitro fertilization (IVF), the EDD is derived directly from the date of embryo transfer (e.g., embryo transfer date + 266 days - embryo age in days). The IVF dating is never altered by subsequent ultrasound biometry.

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