By 32 weeks, a lamotrigine dose is doing a third of the work it did before pregnancy

Design

Prospective, multicentre observational cohort with linear mixed-effects models and model averaging

Population

947 women with epilepsy aged 18-45 in China, 1,638 steady-state trough samples 2019-2025 (451 during pregnancy in 228 women); 64.3% on polytherapy

Primary outcome

Gestational change in antiseizure medication concentration-to-dose ratio by trimester

Effect

Lamotrigine -28.8%, -54.3%, -63.2% by trimester (nadir -65.8% at 32 weeks); levetiracetam -26.2%, -40.1%, -31.0% (nadir -35.5% at 24 weeks); oxcarbazepine metabolite -23.1%, -32.6%, -44.3%; lacosamide -10.8% in the second trimester

Antiseizure drug levels fall in pregnancy and everyone knows it. What has been missing is a number per drug, per trimester, from a large enough cohort to act on. This prospective multicentre Chinese study followed 947 women with epilepsy aged 18-45 between 2019 and 2025, with 1,638 steady-state trough samples - 451 of them during pregnancy - and modelled concentration-to-dose ratios against gestational age.

Lamotrigine moved most. Concentration-to-dose ratios fell 28.8% in the first trimester, 54.3% in the second and 63.2% in the third, reaching a nadir of -65.8% at 32 weeks. Levetiracetam fell 26.2%, 40.1% and 31.0%, bottoming out at -35.5% at 24 weeks - note it recovers slightly in the third trimester, which lamotrigine does not. The oxcarbazepine metabolite fell 23.1%, 32.6% and 44.3%. Lacosamide fell only 10.8%, and only in the second trimester. Perampanel trended upwards but the sample was too small to say more.

Two secondary findings matter as much as the headline. Concomitant antiseizure drugs shifted baseline concentration-to-dose ratios without changing the shape of the gestational decline - so polytherapy alters where you start, not how fast you fall. And interindividual variability was the dominant determinant of concentration overall, larger than pregnancy, weight or co-medication.

That last point is the argument for individual therapeutic drug monitoring rather than a fixed dose-escalation schedule. Get a preconception baseline level while the patient is stable and seizure-free, then measure at least once each trimester and adjust to that individual's own baseline rather than to a population range. Lamotrigine and oxcarbazepine need the closest watching, and in India, where preconception counselling for women with epilepsy is often absent and levels are not routinely available, the practical first step is simply establishing which drugs your laboratory can measure.

• Take a preconception baseline level while the patient is stable - the target is her own level, not the reference range
• Measure at least once per trimester on lamotrigine, levetiracetam or oxcarbazepine
• Expect lamotrigine to need the largest dose increase, and to keep needing it into the third trimester
• Reduce the dose promptly postpartum; the concentrations return as fast as they fell
• Polytherapy shifts the starting concentration, so do not assume a combination protects against the fall

The statistics, in plain English

A concentration-to-dose ratio corrects for dose, so a 63% fall means the same milligram delivers a third of the previous blood level - it is not the same as a 63% fall in efficacy, but it is the quantity you dose against. The confidence intervals are narrow because there are 451 pregnancy samples, so these percentages are reliable as population averages. What they cannot do is predict any individual woman, and the paper says explicitly that interindividual variability outweighed every measured covariate - which is precisely why the answer is monitoring rather than a formula.

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