Pregnancy in underweight women

M R C O G PA R T 2 · S O S P O C K E T C A R D · K N O W L E D G E A R E A 5 – A N T E N ATA L C A R E

Pregnancy in Underweight Women: Implications, Management and Outcomes Burnie R et al. Pregnancy in underweight women: implications, management and outcomes. TOG 2022;24:50–57. doi:10.1111/tog.12792

<18.5 3–4% up to 30% KG/M² DEFINES MATERNAL UNDERWEIGHT UK WOMEN UNDERWEIGHT AT BOOKING WOMEN, REPRODUCTIVE AGE, SUB-SAHARAN AFRICA

DO N' T-M IS S FA C TS

  1. BMI <18.5 kg/m² = underweight; NICE weight-management guidance excludes this group – paucity of evidence-based guidance

  2. U-shaped fetal outcome curve – lowest risk BMI 20–25; main risks with low BMI are ↑LBW and ↑late preterm birth (34– 37/40)

  3. RCOG: BMI <20 = minor SGA risk factor only; consider extra growth scans pragmatically if BMI <19 or poor weight gain

  4. IOM weight gain target for underweight women: 12.5–18 kg (vs 11.5–16 kg normal, 5–9 kg obese)

  5. Low BMI is protective against pre-eclampsia, gestational hypertension, gestational diabetes, LGA, caesarean section

  6. "Hamster sign" (parotid enlargement) and finger-joint calluses suggest self-induced vomiting – screen for ED at first antenatal visit

  7. Substance misuse in up to 50% of ED – use Whooley questions + AUDIT-C; hypokalaemic alkalotic renal failure = extreme AN emergency

  8. Low-BMI women may need safeguarding referral as vulnerable adults; Healthy Start vouchers/vitamins available in UK for eligible women

INVESTIGATION TRIGGERS AT A GLANCE

Hb <105 g/L → ferritin/B12/folate Low K⁺ → ECG ± CXR BMI <19 → weigh each trimester/monthly

Ketonuria → consider purging/starvation

ABBREVIATION KEY

BMI — body mass index IOM — Institute of Medicine (2009 weight gain guidance)

NICE — National Institute for Health and Care Excellence ED — eating disorder

RCOG — Royal College of Obstetricians and Gynaecologists AN — anorexia nervosa

Source: The Obstetrician & Gynaecologist (TOG), RCOG journal — doi:10.1111/tog.12792

Clinically reviewed by NHS obstetricians and gynaecologists

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