Medical Calculators for Midwives and Maternity Care

Naegele's rule (LMP + 280 days) is the standard calculation assuming a 28-day cycle. ACOG recommends first-trimester ultrasound dating if the LMP-based EDD differs by >7 days from CRL measurement — ultrasound takes precedence. For irregular cycles or uncertain LMP, ultrasound is the primary dating method. The EDD should be established and documented by 20 weeks. At the booking appointment, midwives should confirm menstrual cycle length and regularity to adjust Naegele's rule accordingly (e.g., a 35-day cycle shifts EDD by +7 days).

The Bishop score assesses cervical readiness for labour induction: dilation (0–3), effacement (0–3), station (0–3), consistency (0–2), and position (0–2), total 0–13. Score ≥8 = favourable cervix, high likelihood of successful induction with oxytocin. Score <6 = unfavourable, cervical ripening recommended first (prostaglandins, Foley catheter, or membrane sweep). Midwives performing membrane sweeps typically document Bishop score before and after. A score of 6–7 is borderline — clinical context, parity, and maternal preference guide the decision.

The Edinburgh Postnatal Depression Scale (EPDS) is a 10-item self-report tool completed by the mother. Score ≥10 suggests possible depression; ≥13 indicates likely major depressive disorder. Question 10 (self-harm thoughts) must always be reviewed individually regardless of total score. NICE guidelines recommend EPDS at the 6–8 week postnatal check and again at 3–4 months. A positive screen requires follow-up clinical assessment — not automatic diagnosis. Midwives should offer a supportive, non-judgmental conversation and refer to the GP or perinatal mental health team as appropriate.

Apgar score is assessed at 1 and 5 minutes post-birth across five parameters (each 0–2): Appearance (colour), Pulse (heart rate), Grimace (reflex irritability), Activity (muscle tone), Respiration. Score 7–10 = reassuring; 4–6 = moderate concern, stimulation and supplemental oxygen; <4 = requires immediate resuscitation. If the 5-minute score is <7, repeat at 10, 15, and 20 minutes. The 5-minute score is more predictive of neonatal outcome than the 1-minute score. Document the score in the maternal and neonatal records and communicate to the paediatric team for scores <7 at 5 minutes.

Key gestational age milestones: <24+0 weeks = periviable (specialist neonatology input required for any birth); 24+0–27+6 = extreme preterm (NICU required); 28+0–31+6 = very preterm; 32+0–36+6 = late preterm (close neonatal monitoring); 37+0–38+6 = early term (increased risk vs full term); 39+0–40+6 = full term (optimal outcome); 41+0–41+6 = late term; ≥42+0 = post-term (induction recommended). ACOG recommends offering induction by 41+0 weeks. Midwives should accurately date pregnancy by 20 weeks to ensure appropriate management at these thresholds.

IOM guidelines by pre-pregnancy BMI: underweight (BMI <18.5) = 12.5–18 kg total; normal weight (18.5–24.9) = 11.5–16 kg; overweight (25–29.9) = 7–11.5 kg; obese (≥30) = 5–9 kg. Rate of gain: 0.5–2 kg in the first trimester, then ~0.3–0.5 kg/week (normal weight) in the second and third trimesters. Excessive weight gain increases risk of gestational diabetes, pre-eclampsia, caesarean section, and postpartum weight retention. Inadequate gain is associated with fetal growth restriction. Midwives should offer brief dietary and physical activity counselling when gain deviates significantly from targets.