Fetal programming

Why can a person's risk of heart disease be partly settled before they are born?

Heart disease looks like a disease of accumulated adulthood. Decades of diet, smoking, blood pressure, exercise, luck. Whatever else is true of it, the causes seem to lie in the life a person actually leads.

So consider a finding that does not fit that picture. In the 1980s David Barker noticed that the parts of England and Wales with the highest infant mortality early in the twentieth century were the same places with the highest death rates from ischaemic heart disease sixty years later. Follow-up work on old birth records found that lower birth weight was associated with higher later heart disease. Something about the months before birth appeared to be leaving a mark that was still legible in a sixty-year-old.

The obvious objection is that poor babies become poor adults, and poverty explains both ends. That objection is serious and we will come back to it. But hold the possibility open for a moment and ask a different question: is there any reason a developing body would carry something forward that an adult body could not later revise?

Start with the most ordinary observation about growth. An adult body is in constant repair — skin, gut lining, blood cells, bone — and almost anything you do to it can be partly undone. But some structures are not built continuously. They are built once, during a window, and then the building stops.

The kidney is the clean example. New nephrons — the filtering units — are formed during gestation and that process finishes before term, at around 34 to 36 weeks. After birth, no more are made. Whatever number you finished with is the number you have for life. A fetus that was short of nutrients or oxygen during that window, and that grew slowly as a result, tends to end up at the lower end of the nephron range. Nephron number varies severalfold between healthy people, and a lower count means each remaining unit works at higher pressure — which is a plausible route to higher blood pressure decades later.

Now notice the structural feature that makes this different from ordinary risk. The window closes. There is no later opportunity to add nephrons; the decision, if we can call it that, is not revisited. That is what a path-dependent process is: not merely that early events matter, but that early events foreclose the options, so the later system carries a setting made under conditions that no longer apply.

So what makes the setting? Here we should be careful, because this is where the field's confident-sounding story is actually a hypothesis. The leading idea is that the fetus reads its supply line — nutrients across the placenta — as information about the world it is about to enter, and calibrates accordingly: grow smaller, prioritise the brain over the trunk, build a metabolism thrifty with fuel. If the world it then meets is scarce, the calibration fits. If the world is abundant, the same settings become a liability, and the mismatch shows up as obesity, diabetes and vascular disease. That is the thrifty phenotype, or predictive adaptive response, and it is a good story that is not fully established.

What is better established is a natural experiment. In the Dutch Hunger Winter of 1944 to 1945, a defined population was subjected to famine for a known number of months, and the exposed pregnancies are identifiable by date of conception. Those exposed in early gestation showed worse cardiovascular and metabolic outcomes as adults than their unexposed siblings — and six decades later, researchers found differences in DNA methylation at a growth-related gene in the exposed group. Timing mattered: the same famine had different consequences depending on which trimester it hit, which is exactly what you would expect if different structures have different windows.

Which brings the confounding objection back, and mostly answers it. Sibling comparisons and a famine defined by calendar dates are hard to explain by adult poverty alone. Still, the honest position is that birth weight is a crude summary of a great deal, that residual confounding by maternal genetics and circumstance remains a live argument, and that these effects are modest shifts in risk, not fates.

Two misreadings are worth heading off. The first is genetic fatalism by another name — the idea that fetal programming means your risk is written before birth. It does not. These are shifts in probability laid over everything that comes afterwards; the adult exposures still dominate, and the fetal contribution shows up most clearly in populations, not individuals.

The second is the reverse: that because the effect is modest, small birth weight is simply a marker of disadvantage with no mechanism behind it. Nephron number and its fixed window are a real, countable mechanism, and the sibling and famine designs make pure confounding an uncomfortable explanation.

The useful correction is to stop thinking of a single risk being set and start thinking about windows. Different structures have different closing dates, so the same insult produces different consequences depending on when it lands. And note one more twist the mismatch idea implies: rapid catch-up growth after a constrained pregnancy is itself associated with worse later metabolic outcomes, so the intuitive remedy of feeding a small baby up quickly is not obviously the right one.

• Whether paternal exposures before conception leave comparable marks, and how the evidence there compares.
• How much of the epigenetic story is causal mechanism and how much is a durable trace of something else.
• Why the same reasoning applies to the immune system and to the timing of childhood infections.

Every term the collection defines is gathered in the glossary.