In one line
Periconception care is the window — from roughly three months before conception through early embryogenesis — in which optimising a woman's medical conditions, medications, micronutrients and immunity changes the outcome of a pregnancy she does not yet have; its single defining principle is that the highest-yield obstetric interventions happen before the first antenatal visit, because by the time most women book, organogenesis is already complete.
Mechanism & pathophysiology
The biological case for preconception care rests on a timing mismatch that no amount of good antenatal care can overcome. Organogenesis is largely complete by the end of the eighth post-conceptional week — by ten weeks' gestation in menstrual dating. The structures most vulnerable to teratogenic insult are forming during the very weeks in which a pregnancy is typically unrecognised: the embryo is most susceptible to major structural malformation between roughly the third and eighth weeks after conception, the classic critical period. An intervention that begins at the booking visit at 10–14 weeks arrives after the developmental decisions it could have influenced have already been made.
The neural tube is the cleanest illustration. The neural tube closes by about day 26–28 after conception — before a missed period is confirmed in many women, and well before a first antenatal contact. Folate supplementation cannot reverse a closure that has already failed; it can only support the closure if the substrate is present during the closure. This is why "start folic acid when you find out you are pregnant" is biologically too late for neural-tube defect (NTD) prevention, and why preconception, not antenatal, supplementation is the evidence-based recommendation.
Folate's mechanism is in one-carbon metabolism. Folate (as 5-methyltetrahydrofolate) donates methyl groups for the remethylation of homocysteine to methionine and for de-novo synthesis of purines and thymidylate — the nucleotide building blocks of DNA. Rapidly dividing tissue, such as the closing neural folds, has a steep demand for this one-carbon flux. Methionine in turn feeds S-adenosylmethionine, the universal methyl donor for the DNA and histone methylation that governs which genes are expressed in the early embryo — so a folate-deficient periconception state is both a nucleotide-supply problem and an epigenetic one, which is part of why folate's reach extends beyond NTDs to a smaller reduction in some congenital heart and orofacial-cleft defects. Where folate is limiting, or where a genetic or pharmacological block raises that demand (anti-folate anticonvulsants, the methylenetetrahydrofolate reductase polymorphisms that reduce conversion to the active form, the hyperglycaemic milieu of poorly controlled diabetes), the failure rate of neural-tube closure rises. Supplemental folic acid raises tissue folate above the threshold at which closure becomes substrate-limited. The mechanism is preventive saturation, not repair, which is exactly why the dose has to be on board before day 28 — and why fortification, which keeps the whole population's red-cell folate elevated continuously, protects pregnancies that were never planned.
A wider mechanistic frame sits behind all of this. The developmental origins of health and disease hypothesis — the Barker work on fetal programming — holds that the intrauterine environment, set in motion by maternal nutrition and metabolic state from conception onward, programmes the offspring's later risk of cardiometabolic disease. Maternal hyperglycaemia, obesity, undernutrition and micronutrient deficiency at the point of conception are not neutral background; they are early inputs into a developmental trajectory. The periconception window is therefore not only about avoiding discrete catastrophes (an NTD, a cardiac defect) but about the metabolic setting in which the conceptus begins. That reframes preconception care from a checklist of single risks into an attempt to start the pregnancy in the best possible physiological state.
The corollary for diabetes makes the point quantitatively. Maternal hyperglycaemia in the first trimester is teratogenic in a dose-dependent way: a first-trimester HbA1c above 10% (86 mmol/mol) carries congenital-malformation rates reported around 20–30%, against a background population rate of 2–3%. Glucose crosses the placenta freely and, in the unregulated embryonic milieu, drives oxidative stress and disordered gene expression in the very tissues differentiating during weeks three to eight; the malformations that result — cardiac outflow lesions, neural-tube defects, the near-pathognomonic caudal regression sequence — track the period of organogenesis precisely. The risk is graded, not all-or-nothing: every increment of HbA1c toward normal lowers it, which is why "near target" still helps when "at target" is not achievable. Glycaemic optimisation before conception is the only intervention that addresses this, because by booking the damage, if any, is done.
The same logic generalises to every teratogen and to maternal weight. A drug's danger depends on whether the woman is taking it during the window of susceptibility of the organ it affects, and most teratogenic exposures that matter occur before pregnancy is recognised — which is the unifying argument of the whole topic. Obesity raises the conceptus's risk of NTDs and cardiac defects (in part through relative folate insufficiency and the metabolic environment), and underweight and micronutrient deficiency shift the developmental programme in the opposite, growth-restricted direction. None of these is correctable from the booking visit; all are addressable from a preconception one.
Assessment
A structured preconception assessment is a systematic search for the modifiable risks that, left to the booking visit, would be too late to modify. It is opportunistic in most South African settings — taken at a contraception consultation, a chronic-disease visit, a postnatal review, a termination-of-pregnancy contact — because dedicated preconception clinics are scarce and the unplanned-pregnancy rate is high.
Medical conditions — the optimisation list.
- Diabetes (pre-existing type 1 or type 2): the highest-yield single condition. Assess glycaemic control (HbA1c), end-organ status (retinopathy, nephropathy with albumin:creatinine ratio and eGFR, neuropathy), and current therapy. A woman on an oral agent other than metformin, or on an ACE-inhibitor/ARB for nephroprotection, needs a planned medication change. The target is preconception HbA1c <48 mmol/mol (6.5%) where achievable without disabling hypoglycaemia, and a strong recommendation to defer pregnancy while HbA1c is >86 mmol/mol (10%). This builds on Diabetes in pregnancy.
- Chronic hypertension: identify and stop teratogenic antihypertensives. ACE-inhibitors and ARBs must be switched off — first-trimester exposure is associated with congenital malformation, and second/third-trimester exposure causes the fetopathy syndrome (renal failure, oligohydramnios, pulmonary hypoplasia, skull hypoplasia). Substitute a pregnancy-compatible agent before conception. See Hypertension in pregnancy antihypertensives.
- Epilepsy: review the anticonvulsant. Sodium valproate carries a major-malformation rate around 10% and a 30–40% rate of neurodevelopmental disorder, and is contraindicated in women of childbearing potential outside a pregnancy-prevention programme; switching to a lower-risk agent (and to monotherapy at the lowest effective dose) is a preconception, not antenatal, task, because the change must be stable and seizure-controlled before conception. Anti-folate anticonvulsants also dictate high-dose folate.
- Thyroid disease: confirm euthyroidism; hypothyroid women typically need a dose increase early in pregnancy and benefit from optimisation beforehand. See Thyroid disorders in pregnancy.
- Cardiac disease: the consultation in which a woman with significant structural or pulmonary-vascular disease is counselled about the maternal risk of pregnancy — including the conditions in which pregnancy is contraindicated — belongs preconception, not at booking. Risk-stratify (the modified WHO classification) and refer. See Cardiac disease in pregnancy.
- Renal disease: baseline eGFR, proteinuria and blood pressure stratify the risk of accelerated decline and of superimposed pre-eclampsia, and frame the conversation about timing. See Renal disease in pregnancy.
- Autoimmune disease / antiphospholipid syndrome: disease should be quiescent on pregnancy-compatible therapy before conception (switching off methotrexate and mycophenolate, which are teratogenic); APS dictates a planned aspirin ± heparin regimen. See Sle and antiphospholipid syndrome.
- Mental health: screen for existing illness — including a history of bipolar disorder or postpartum psychosis, which carries a high recurrence risk in a future puerperium and reshapes the whole peripartum plan — and review psychotropics for the risk/benefit of continuation. The reflex to stop every psychotropic before pregnancy is a trap: abrupt discontinuation of effective treatment risks relapse, and an unwell mother is itself an adverse exposure for the pregnancy. The preconception task is a deliberate medication review (for example, the specific concerns around sodium valproate as a mood stabiliser and around certain agents in the first trimester) and a shared decision about continuing, switching or adjusting, made before conception when there is time to monitor the change.
- Haemoglobinopathy: in at-risk populations, identify carriers — this is the entry point to couple/genetic counselling.