Most women will have been made aware they have a “biological clock” and that it’s ticking. Most know the older women become the harder it is for them to conceive. But most probably don’t know it’s because women are born with a limited supply of eggs, and eventually they will run out.

Eggs are made when women themselves are in the womb. By 20 weeks’ gestation the tiny developing ovaries in a human fetus contain about five million eggs (the technical names for which are gametes, oogonia or oocytes). This is the maximum number of eggs a female will ever have, because new ones stop being made after this time.

For reasons that are still unclear, more than two-thirds of these newly made eggs degenerate in the following months, leaving a much smaller supply of eggs at birth. Egg numbers at birth range anywhere from half to one million.

The eggs present at birth constitute the only supply of eggs a woman will have in her lifetime. This stockpile of eggs, which is called the ovarian reserve, is housed in structures called primordial follicles.

Although quite a few follicles (30-40) start to develop in waves before each ovulation, usually only one follicle makes it to ovulation to release an egg. The rest degenerate over the six-to-eight-week development phase. So a woman actually ovulates only about 400 eggs during her reproductive life. This is about 1% of the pool of follicles ever produced.

By the age of 30, women will have on average only 12% of the number of eggs they had at birth. This number is still sufficient to support fertility for the next few years, provided the ovary is not subjected to external influences such as cancer drugs or serious ovarian surgery.

But what about IVF?

IVF can lead to pregnancy, but only if the eggs are healthy. IVF cannot make an unhealthy egg healthy again, given our current state of knowledge and techniques.

The chances of establishing a pregnancy with the assistance of IVF as women age is well documented. It can be 30-45% in women under 38 and has been shown to decline to less than 10% after 42 years of age.

What about freezing eggs?

Egg freezing can be performed to preserve a number of mature eggs for later use. This involves a process of ten to 12 days of hormone administration to stimulate development of multiple follicles.

The number of follicles that develop depends on the age of the woman and her intrinsic fertility. The average number of eggs collected and subsequently frozen for a 35-year-old woman is about ten.

Although 90-95% of eggs make it through the thawing process, we would still expect only one to three good embryos to develop from that group of ten eggs. This is the same as fresh eggs – about 50% fertilise and then fewer develop to make good embryos.

Both the processes of embryo creation and the pregnancy potential of embryos are similar from fresh and from frozen eggs. But the “normal” loss that occurs, as the eggs are fertilised, made into embryos and then develop, means that at most the process of egg freezing will offer a small finite number of additional opportunities for a woman to conceive in the future.

Can new technology make new eggs?

A recent report shows new healthy eggs can be made from stem cells. Stem cells are present in human embryos, as embryonic stem cells, and in most organs including the ovary. Alternatively, an induced form of stem cells can be obtained by treating mature cells with a cocktail of reagents in the laboratory.

The procedures required to create new eggs out of stem cells are very complex and still experimental. There are ethical issues, such as the need to destroy a human embryo to obtain embryonic stem cells, and further experiments will be necessary to show there are no genetic or fertility problems with subsequent generations.

It will require much more research to establish the safety and efficacy of the procedures before they are allowed into clinical use.

Women do not have an unlimited supply of eggs, but it may be possible in the future to create healthy eggs from stem cells in the laboratory. Until such a time, it’s essential all women and men understand the limitations of the fertility of women and plan their lives to take full advantage of the fertile lifespan should they choose to have children.

Food cravings are very common. One study of more than 1,000 people revealed 97% of women and 68% of men experienced cravings. Food cravings occur more commonly later in the day, with an average of two to four craving episodes per week.

Nutritional deficiencies

It has been long thought that food cravings were due to the body’s effort to correct nutritional deficiencies or food restrictions.

Under this theory, a craving for a juicy steak might indicate the body’s need for iron or protein. A craving for chocolate may indicate that people lack phenylethylamine, a chemical that has been associated with romantic love. Phenylethylamine is found in significant amounts in chocolate.

Nutritional deficiencies are linked to food cravings in certain situations. Pica is an unusual behaviour where people crave non-food substances such as ice, clay or raw starch. Pica behaviour is sometimes found in conjunction with micronutrient deficiencies such as zinc.

Deficiencies in vitamins may potentially result in food cravings. A severe deficiency of vitamin C led to scurvy in maritime explorers who did not have ready access to fresh fruit and vegetables during their long sea voyages. A British chaplain who wrote about the accounts of sailors suffering from scurvy reported they had intense cravings for fruit and when they finally were able to eat it they experienced “emotions of the most voluptuous luxury”.

In general, however, there is no real evidence to link our common food cravings with nutritional deficiencies.

Firstly, food cravings have been shown to decrease during weight-loss diets rather than increase, as might be expected.

In one study, a group of obese people was restricted to a very low-calorie diet over a 12-week period. Only meat, fish or poultry was allowed and all other foods were forbidden. Their cravings for low-fat, high-protein foods and complex carbohydrates decreased markedly on the diet. There was no reported increased craving for forbidden foods.

Restriction of certain types of foods also appears to decrease food cravings rather than increase them. A study of low-carbohydrate and low-fat diets in obese adults found that restricting carbohydrates resulted in decreased food cravings and restriction of fats decreased their craving for high-fat foods.

If the nutritional deficiency theory were to be true, this does not explain why some foods that are richer in nutrients lead to generally less cravings than other foods. Cheddar cheese and salami, for example, have much higher levels of phenylethylamine than chocolate but not nearly the same intensity of craving.

What causes food cravings?

Food cravings are believed to come from a mix of social, cultural and psychological factors. In North America, chocolate is the most-craved food, but this is not the case elsewhere. In Egypt only 1% of young Egyptian men and 6% of young Egyptian women reported craving chocolate. Japanese women are more likely to crave rice and sushi, reflecting the influence of traditional food products and culture.

The nature of the relationship between specific foods and cravings is important. Food cravings can develop from matching consumption of certain foods with hunger, suggesting a conditioning response. In one study, some participants were assigned to eat chocolate only when hungry (between meals). They developed greater cravings for chocolate after two weeks than other participants who ate chocolate exclusively when full (just after meals).

A theory of food cravings that includes the biological, psychological and social aspects suggests they can arise from matching food intake with other conditions such as emotional states (“stress eating”). Food cravings have been shown to be linked to higher levels of stress.

Evidence is also emerging that suggests our gut microbes (the bacteria in our guts) influence our food cravings.

Controlling food cravings

As described earlier, restricting certain types of foods can decrease food cravings. In the study of obese patients with restriction of carbohydrates and high-sugar foods it was found that food preferences and to a lesser extent food cravings were suppressed during a two-year period, suggesting long-term benefits.

Committing to implementing change is not easy. Cognitive techniques such as mindfulness can help.

Researchers gave 110 self-identified chocolate cravers a bag of chocolates each to carry around for a week. They instructed half the group in “cognitive restructuring”, a technique that involves challenging inaccurate thoughts and replacing these with more accurate ones.

The other half of the group was taught a mindfulness-based technique – “cognitive defusion”. Participants were asked not to change their thoughts but simply to notice their thoughts and to visualise themselves as different from their thoughts.

At the end of the study participants in the defusion group were more than three times more likely to abstain from chocolate than participants in the restructuring group.

Defusion interventions work to resist food cravings by creating a sense of distance from them rather than trying to eradicate and replace them.