A concentration measured in the second trimester is only useful if you know what that woman's concentration was when her epilepsy was controlled and she was not pregnant. Without that comparator, a level sits against a population reference range that was never her therapeutic target, and the decision to increase the dose becomes guesswork.
The practical difficulty is timing. By the time a pregnancy is confirmed, first-trimester clearance changes are already underway, so the baseline has to be taken before conception. That means taking it at a routine review in any woman of childbearing potential who is not using reliable contraception — not at a preconception clinic that many women never attend.
One level, filed in the notes with the date and the dose, converts every subsequent pregnancy measurement from an isolated number into a comparison. It costs one blood test taken at a visit that was happening anyway.
- Take and record a trough level with the dose and date at a routine review, not only at preconception counselling
- Repeat it if the regimen changes, so the filed baseline matches the current drug
- Note the target explicitly as 'her level when controlled' rather than relying on the laboratory range
- Carry the baseline forward into the antenatal record so the obstetric team has it
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