Prescribing in Pregnancy, Breastfeeding and Childhood

Pharmacology — Prescribing in Pregnancy and Childhood, NMC MBBS licence examination syllabus (Nepal Medical Council).

Prescribing in pregnancy, breastfeeding and childhood

Two groups where the usual rules do not apply and the consequences of getting it wrong last a lifetime.

Most prescribing errors are recoverable. In these two groups they frequently are not. A teratogenic exposure during organogenesis causes a permanent structural abnormality, and a tenfold dosing error in a small child can be fatal where the same proportional error in an adult would not be.

What makes this difficult is that both groups are systematically under-represented in the evidence base, so the information available is thinner exactly where the stakes are highest. That is why the discipline here is not memorising lists but building two habits: asking before prescribing, and checking a current source rather than relying on memory.

🩺 Where this lives: The period of greatest teratogenic risk is often before a woman knows she is pregnant. Organogenesis is well under way by the time a period is missed and a test is positive, which means the protective act is not asking a pregnant woman what she is taking — it is asking every woman of childbearing age, before writing the prescription, whether she could be pregnant. That question costs nothing and is the only one that arrives in time.

💡 A note on lists. This chapter deliberately gives no doses, no weight-based formulae, no gestational thresholds and no list of drugs classified as safe or unsafe. Pregnancy and lactation safety classifications differ between sources and are revised, and a list in a revision resource invites use without checking — which in this group causes the most harm. Take every decision from a current national formulary or specialist source. General prescribing safety is covered in the Prescribing Safety chapter, and the specific pregnancy-contraindicated classes appear in the Cardiovascular Drugs, Thyroid and Hypertension chapters.

The two opposite errors

Both reflexes are wrong, and the second is the one clinicians feel virtuous about. Stopping everything when a woman becomes pregnant looks cautious, but untreated maternal illness harms the fetus too — uncontrolled epilepsy, asthma, diabetes, thyroid disease, severe infection and severe mental illness all endanger both mother and baby. The correct approach weighs the risk of the drug against the risk of the untreated condition, drug by drug, with specialist input where the condition is serious.

HOW TO ACTUALLY MAKE THE DECISION BEFORE PRESCRIBING TO ANY WOMAN OF CHILDBEARING AGE Ask whether she could be pregnant, and whether she is planning pregnancy. Neither question is intrusive when framed as routine. WHEN A PREGNANT WOMAN IS ALREADY ON TREATMENT Do NOT stop reflexively. Ask: What happens to HER and to the FETUS if this condition is untreated? Is there a better-established alternative for pregnancy? Can the dose be optimised rather than the drug stopped? Does this need specialist input before any change? WHEN A WOMAN ON LONG-TERM TREATMENT PLANS PREGNANCY This is the ideal situation, because changes can be made BEFORE conception rather than after organogenesis has begun. Pre-conception review is far more valuable than an urgent decision at eight weeks. AND REMEMBER THE POSITIVE PRESCRIPTIONS FOLIC ACID before conception to reduce neural tube defects — started before pregnancy, not after it is confirmed (see the gynaecology chapter). Iron and other supplementation per the antenatal programme. Treatment of anaemia, infection and hypothyroidism, all of which benefit the fetus. DOCUMENT the reasoning. A decision that was carefully weighed should look like one in the notes.

Timing of risk

💡 Exam angle: the examinable structure is that what a drug does depends on when it is given. Very early exposure tends to be all-or-nothing. Exposure during organogenesis in the first trimester carries the risk of structural malformation. Later exposure affects growth and function — including effects on fetal kidneys and brain, and effects on the neonate around delivery. A drug that is acceptable in one trimester may not be in another, in either direction.

Children are not small adults

WHERE PAEDIATRIC PRESCRIBING GOES WRONG THE WEIGHT Doses are calculated by weight, so an inaccurate weight produces an inaccurate dose. Estimating or recalling a weight rather than measuring it is a leading contributor to serious paediatric dosing error — and the child has grown since the last admission. THE ARITHMETIC Tenfold errors are the characteristic paediatric harm, and they come from decimal points, unit confusion (milligrams and micrograms), and trailing zeros. Write clearly, state the weight used on the chart, and have calculations independently checked where local practice requires it. THE PHYSIOLOGY, WHICH CHANGES WITH AGE Newborns have immature hepatic metabolism and renal excretion, so drugs are cleared slowly. Body water is proportionally higher and fat lower, which alters distribution. Protein binding differs. The blood-brain barrier is more permeable in the newborn. Older children may clear some drugs FASTER than adults relative to weight. THE FORMULATION Can the child actually take it? A tablet that cannot be swallowed is not a treatment, and liquid preparations introduce their own measurement errors. AND THE LICENCE Many paediatric uses are outside the product licence simply because children are under-represented in trials. That is not a reason to withhold treatment — it is a reason to follow paediatric formulary guidance rather than adult labelling.

Specific hazards in children

Two points deserve emphasis. The newborn is the most vulnerable patient in medicine for prescribing purposes — immature metabolism and excretion, a more permeable blood-brain barrier, and minimal reserve. And medicines kept at home are a leading cause of childhood poisoning, which makes storage advice part of the prescription rather than an afterthought, particularly where a household contains both an elderly person on many drugs and a small child.

Breastfeeding

💡 Exam angle: the expected answer is usually that breastfeeding can continue. Most drugs appear in milk in very small quantities, and stopping breastfeeding has real costs — nutritional, immunological and relational — that are frequently ignored when a drug is prescribed. But the exceptions are genuine: sedatives, cytotoxics, radioiodine and certain other agents, and a much lower threshold for concern in a preterm or unwell infant. The rule is to check a current source rather than to assume in either direction.

Clinical reasoning: four presentations

🔍 Case 1 — everything stopped at once

PresentationA woman with well-controlled epilepsy discovers she is pregnant. She is advised to stop her antiepileptic immediately because of possible fetal risk. Two weeks later she has a prolonged seizure.

ErrorThe cautious-looking reflex.

ReasoningUncontrolled seizures endanger both mother and fetus, and abrupt withdrawal is itself a recognised precipitant. The decision required weighing the drug's risk against the risk of uncontrolled epilepsy, with specialist input.

AnswerNever stop an antiepileptic abruptly in pregnancy. Refer urgently for specialist review, and ideally undertake such changes before conception rather than after.

🔍 Case 2 — the question not asked

PresentationA 26-year-old is prescribed a drug known to be teratogenic. Nobody asks whether she could be pregnant. She is six weeks pregnant and does not yet know.

Missed stepOne question, before the prescription.

ReasoningOrganogenesis is well under way before a pregnancy is usually recognised, so asking after the fact is too late. The protective act is asking every woman of childbearing age beforehand.

AnswerAsk routinely before prescribing, check the drug against a current source, discuss contraception where a teratogenic drug is genuinely required, and seek specialist advice about this exposure now.

🔍 Case 3 — a weight from memory

PresentationA child is prescribed a weight-based drug using the weight recorded at an admission a year ago, because the ward scales are in use. The dose is calculated in a hurry and not independently checked.

Two hazardsAn outdated weight and an unchecked calculation.

ReasoningChildren grow, so last year's weight is wrong. Tenfold errors from decimal points and unit confusion are the characteristic paediatric harm, and they are caught by independent checking.

AnswerWeigh the child, record the weight used on the chart, write the calculation clearly, and have it checked. Recalculate at every admission.

🔍 Case 4 — told to stop feeding

PresentationA breastfeeding mother needs treatment for a common condition. She is told to stop breastfeeding for the duration "to be safe", without anyone checking a source. She switches to formula and does not resume.

CostA real loss for an assumed risk.

ReasoningMost drugs appear in milk in very small amounts, and the nutritional, immunological and relational costs of stopping are substantial and often permanent once formula is established.

AnswerCheck a current source before advising against breastfeeding, choose a compatible agent where one exists, and watch the infant for drowsiness or poor feeding rather than stopping pre-emptively.

Commonly confused

Confusion	The distinction	Why it matters
Prescribing blindly vs asking first	Ask before, not after	Organogenesis precedes the pregnancy test.
Stopping everything vs weighing risk	Untreated illness also harms	The cautious reflex causes harm.
First vs later trimester exposure	Malformation versus growth and function	Safety differs by timing in both directions.
Pre-conception vs post-conception change	Changes are best made before	By eight weeks the window has passed.
Estimated vs measured weight	Children grow between admissions	The dose is only as good as the weight.
Adult vs paediatric handling	Immature clearance in the newborn	Not a scaled-down adult.
Unlicensed vs unsafe	Children are under-represented in trials	Follow paediatric formulary guidance.
Assuming vs checking in lactation	Most drugs are compatible	Stopping needlessly has real costs.

Rapid revision

MUST-KNOW FACTS 1. Errors in these groups are frequently PERMANENT. 2. Both groups are UNDER-REPRESENTED in the evidence base. 3. ASK "COULD YOU BE PREGNANT?" BEFORE PRESCRIBING. 4. The riskiest period is often BEFORE she knows she is pregnant. 5. ERROR ONE: prescribing without asking. 6. ERROR TWO: stopping everything out of caution. 7. UNTREATED MATERNAL ILLNESS ALSO HARMS THE FETUS. 8. Uncontrolled epilepsy, asthma, diabetes, thyroid disease, infection. 9. NEVER stop an antiepileptic abruptly in pregnancy. 10. Weigh drug risk against the risk of the untreated condition. 11. Make changes BEFORE conception where possible. 12. FOLIC ACID before conception, not after confirmation. 13. VERY EARLY exposure is largely all-or-nothing. 14. ORGANOGENESIS — first trimester — risks STRUCTURAL MALFORMATION. 15. LATER exposure affects GROWTH and FUNCTION, and the neonate. 16. Safety differs by trimester in BOTH directions. 17. CHILDREN ARE NOT SMALL ADULTS. 18. Dose by WEIGHT — and MEASURE the weight, do not estimate it. 19. Recalculate at EVERY admission; the child has grown. 20. TENFOLD ERRORS come from decimal points and unit confusion. 21. Write clearly, state the weight used, have it checked. 22. NEWBORNS have immature metabolism and excretion. 23. Higher body water, altered protein binding, permeable blood-brain barrier. 24. Older children may clear some drugs FASTER relative to weight. 25. Can the child actually take the formulation? 26. Many paediatric uses are UNLICENSED — that is not the same as unsafe. 27. MEDICINES AT HOME are a leading cause of childhood poisoning. 28. BREASTFEEDING: most drugs appear in milk in tiny amounts. 29. Stopping breastfeeding unnecessarily has real and lasting costs. 30. Exceptions: sedatives, cytotoxics, radioiodine, some antiepileptics. 31. Lower threshold for concern in a PRETERM or ill infant. 32. Watch the infant for drowsiness and poor feeding. 33. CHECK A CURRENT SOURCE — never assume in either direction.

💡 Exam angle: two habits answer nearly every question here — ask about pregnancy before prescribing rather than after, and check a current source rather than deciding from memory. The wrong answers are the two reflexes: prescribing without asking, and stopping everything to be safe.

Syllabus points

The two opposite errors in pregnancy
Why untreated illness also harms
Asking before prescribing
Pre-conception review
Folic acid and positive prescriptions
All-or-nothing early exposure
Organogenesis and malformation
Later growth and functional effects
Weight-based dosing and measuring the weight
Where tenfold errors come from
How handling differs in the newborn
Formulation and unlicensed use
Medicines at home and childhood poisoning
Breastfeeding compatibility and exceptions

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