New research reveals the extraordinary impact that your mother's diet at the time of your conception has on the rest of your life, writes Michael Mosley.
A couple of months ago, I found myself in a small village in Keneba, in The Gambia, chatting to a perky 90-year-old, Karamo Touray, surrounded by his many children and grandchildren. Apart from a sore toe, he said he was in good shape and he attributed the fact he had enjoyed such a long and healthy life to the will of Allah.
I suspect that the time of year when he was conceived may also have played a role.
A team from Britain's Medical Research Council, which has been collecting data on births, marriages and deaths in Keneba since the 1940s, discovered some years ago that in this part of The Gambia when you are conceived makes a huge difference to your chances of dying prematurely.
If you are conceived in, say, January and born in September then, as an adult, you are seven times more likely to die in young adulthood than someone conceived in September and born in June.
So the effect is big, very big.
Now the reason this happens has nothing to do with astrology and an awful lot to do with the weather, and therefore, what your parents were eating at the time you were conceived. The Gambia has an unusual and very stable weather pattern. July to November is known as the wet season because it rains almost all the time. The other months are largely dry.
During the dry season people have plenty of couscous and rice to eat, and these grains form the major part of their diet. During the rainy season there are fewer calories around (these are known as the Hungry Months) but, thanks to the rain, there are a lot more leafy green vegetables to eat.
And it turns out, certainly in The Gambia, that the amount of leafy green vegetables your mother (and possibly your father) are eating around the time of your conception can have a big impact on the rest of your life.
What really surprised me is that not only are the effects so profound, but that they don't kick in for many years. Up until the age of 15 there's no discernible difference between the children. After that, however, the differences, as I described earlier, become striking, even shocking.
So, what's going on?
The fact that a mother's diet during pregnancy can have long and lasting effects on the health of her child has been known for some time. One of the most dramatic examples of this is the Dutch Famine study.
At the end of World War Two, the Germans blockaded parts of the Netherlands in retaliation against a rail strike called by the Dutch government. By the time the blockade was lifted, winter had begun and it was almost impossible to get food in. For months on end many people had to live on starvation diets. The famine was only ended when the Allies liberated Europe.
Thousands of people died during this famine. The Dutch Famine Birth Cohort Study was set up afterwards, the purpose of which was to see what would happen to the babies of the pregnant women caught up in the famine.
What they discovered is that if you were a young embryo at the time of the famine then you were twice as likely to develop heart disease in later life. You were also far more prone to schizophrenia, obesity, diabetes, cancer and stress-related illnesses.
Worryingly, there's evidence that the effects persist into the next generation. Not only the children but the grandchildren of a woman caught up in the famine went on to have worse health in later life.
On the positive side, what this suggests is that improving the diet of pregnant women would not only improve the lives of their children but of their grandchildren. Or as the authors of the study cautiously put it: "This may imply that improved maternal nutrition during gestation may benefit the health of many generations to come."
Like the people in The Gambia, the impact of the Dutch Famine on the later lives of the children who were caught up in it are probably the result of changes to the genes, changes that occurred in the womb.
Experiments in animals have shown that it is possible to make the genes in an embryo more active, or turn them off entirely, simply by varying their mother's diet.
It would obviously be unethical to do this to people, but the studies done in The Gambia certainly provide compelling evidence that these so-called "epigenetic changes" may also happen in humans in response to a change in diet. That if, during very early development, a mother eats a diet rich in leafy green vegetables, then this will change forever just how active some of her child's genes are.
In happens through a process called methylation and researchers in The Gambia have recently shown that babies conceived in the wet season have very different levels of activity of a particular gene that's important for regulating the immune system.
As Matt Silver, part of the MRC team, says: "Variation in methylation state in this gene could affect your ability to fight viral infections and it may also affect your chances of survival from cancers such as leukaemia and lung cancer."
If you are thinking of having a baby, then eating lots of leafy green vegetables, which are rich in B vitamins and folates, is certainly a good thing to do. Folic acid supplements are also recommended to reduce the risk of neural tube defects.
Countdown to Life, the untold story of your nine months in the womb and how this impacts on your later life is on BBC Two at 21:00 BST on Monday 14 September.