Some people will always be punctual, while others are perpetually late - and our bodies are no different. A recent study by the US National Institute of Environmental Health Sciences found that “due dates” are largely pointless, because the length of pregnancy can vary dramatically from woman to woman, with very few actually giving birth on their predicted day.
So what about the other crucial processes in life - how long should they last? Experts tell all...
DIGESTION: 24 hours
That sausage roll you’ve just scoffed will be in your stomach within three seconds of swallowing it, but the waste won’t be excreted until up to a day later.
As soon as it reaches the stomach, it’s ground into tiny particles with the help of stomach acid, at a rate of about three to four calories a minute, says Dr Anton Emmanuel, consultant gastroenterologist at University College Hospital, London.
“A larger meal with a high fat content will be harder for the acid to break down, but a 600-calorie roast dinner, for instance, will be in your stomach for about two to three hours.”
Next, the food travels through the small intestine where it’s further broken down, and nutrients absorbed into the bloodstream, before entering the colon after another two to three hours, where water and salt are extracted.
“For the next 20-odd hours, it’s working its way through your colon so the whole thing takes about 24 hours,” says Dr Emmanuel.
“But there’s a huge variation in what normal is - anything from 16 to 30 hours for a standard meal is considered healthy.”
Individual foods vary in time taken to pass through. A study from the University of Hawaii found it takes up to three days to digest a complex food such as a hamburger, whereas fruit can take less than an hour. Digestion is a bit slower in women than men - possibly because the gut hormones that aid digestion are less powerful in women, says Dr Emmanuel.
Digestion also slows down with age as the system becomes less effective; drugs such as blood pressure medications and antidepressants can cause the bowel to slow right down. Mood plays a part, too, he says. “Anxiety can slow digestion and that’s probably something that’s adapted - if you’re stressed you need more calories on board to cope with things.”
Certain conditions speed the process up - irritable bowel syndrome being the most common, causing the process to take less than 16 hours. Inflammatory bowel diseases, such as ulcerative colitis and Crohn’s disease, and an overactive thyroid, can have this effect, too (an underactive thyroid, however, slows things down). This may be a factor in people with these conditions suffering a lack of vitamins and minerals and losing weight.
WOUND HEALING: Up to two years
It takes two years for the tissue to recover fully following a deep wound such as one sustained through surgery - although the actual healing process starts within seconds of the damage occurring, according to a study from Columbia University.
In less than one second, the blood vessels leading to the wound tighten to reduce blood flow, and platelets are dispatched to help the blood clot and plug the opening.
Various substances, including the mineral calcium, vitamin K, and a protein called fibrinogen, form a type of net to hold the platelet plug in place, and a scab starts to form.
With the wound sealed, about three weeks after the injury, scar tissue forms. Over the next two years or so, the scar tissue will become stronger, although it will look faded - it will eventually have about 80 percent of the strength of the original skin.
Various conditions can inhibit wounds healing, including diabetes, as can taking drugs such as corticosteroids, ibuprofen and aspirin, possibly because they weaken the net that holds the platelet plug in place.
Vitamin C is vital for wound healing and so a lack of this could result in a cut taking longer to heal. Age and conditions that reduce the amount of oxygen in the blood, such as sleep apnoea, can also slow healing.
WEIGHT GAIN: Three hours
This is the time it takes fat to reach the waistline after a big meal - dietary fat enters the blood around an hour after a meal, and after three hours, much of it is found in adipose tissue, the fatty tissue found mostly around the waist, according to research from Oxford University.
In a meal containing 30g of total fat (such as a chicken tikka masala or beef lasagne) there will be two to three teaspoons of fat in tissue around the waist after about three hours. That is temporary storage and will be used as an energy source - unless we eat too much, when it will remain, and accumulate, so we put on weight.
ORGASM: 20 mins for women, three for men
An orgasm can last anything from a few seconds to half a minute, studies show - however, the time it takes to reach that point varies: for women, an average of ten to 20 minutes; for men, two to three minutes.
But Professor John Studd, a gynaecologist from the London PMS and Menopause Clinic, questions the findings, saying there are too many variables to give an average length of time.
“Orgasms - and libido - are a mixture of head, heart and hormones. There are no hard and fast rules.”
REPLACE A PINT OF BLOOD: Up to six weeks
After losing or donating a pint of blood, it takes around 24 hours for the watery part of the blood, the plasma, to be replaced.
But it takes the red blood cells -which carry oxygen - much longer, about four to six weeks. The process starts with cells in the kidneys - the peritubular cells - sensing the oxygen level in the blood has dropped. They start secreting erythropoietin, a protein that travels to the bone marrow to trigger the production of stem cells, the building blocks needed to make blood cells.
Men who donate blood have to wait a minimum of 12 weeks before doing it again; women, 16 weeks. This is because men normally have more iron stores, needed to produce red blood cells.
CONCEPTION: 30 minutes
It takes as little as half an hour for a sperm to travel up the cervix and fertilise an egg in the fallopian tubes.
The egg releases a hormone that attracts the sperm, which reacts in under a second, according to research from the University of California. Fertility problems can occur if the egg doesn’t release enough of the hormone, or if the sperm cells don’t recognise it.
Once an egg is fertilised, gestation time can vary massively, as researchers at the US National Institute of Environmental Health Sciences have found - anything from 37 to 42 weeks.
The researchers found older mothers, and women who had themselves been heavier at birth, were more likely to have longer pregnancies, though it’s not clear why.
Delivery of a baby is triggered when the foetal lungs fully develop - this sends a signal to the uterus to start contracting. However, several factors can affect the timing, says Dr Victoria Beckett, of the Royal College of Obstetricians and Gynaecologists.
“If the womb is very big - for example, if a woman is having twins - or in diabetic pregnancies, or if there’s infection around the womb, the signal can go off early.”
Although babies born between 37 to 42 weeks are normally healthy, a study published last year in the British Medical Journal found those born at 37 to 38 weeks are more likely to have health problems such as asthma and gastrointestinal disorders.
Meanwhile, those born after 42 weeks (who are usually induced) are more at risk of behavioural disorders such as Attention Deficit Hyperactivity Disorder (ADHD), according to Dutch research, perhaps because these babies are often bigger, so the placenta can’t provide enough oxygen and nutrients.
NAIL GROWTH: 2 to 4mm a month
A study by the University of North Carolina in 2009 found that people’s nails now grow by 3.55 mm a month, compared with 3 mm a month in 1938 and 3.06 mm in the Fifties.
The modern diet, rich in protein and nutrients, is thought to be responsible. (Slow-growing nails or nails that break easily can be a sign of poor diet.)
The study also showed the middle finger has the fastest-growing nail, the little finger the slowest (just 3.08mm a month). This may be because longer fingers that are used more have better blood circulation, aiding growth.
For the same reason, toenails grow more slowly than fingernails - big toenails by around 2mm a month.
Men’s fingernails grow more quickly than women’s, possibly due to hormones, and nails grow faster in the summer, because vitamin D, made by the body using sunlight, aids growth.
PASSING WATER: One hour
On average, it takes an hour for liquid to be absorbed through the gut into the bloodstream, then filtered by the kidneys and sent to the bladder as urine, says Rowland Rees, consultant urological surgeon at the Royal Hampshire County Hospital, Winchester.
If coffee seems to whizz through you that’s not because it speeds up the fluid’s progress through the body; instead it stimulates the bladder, making you want to go more frequently. This is different from alcohol, which is a diuretic, so you produce more urine and need to go more often.
The bladder should hold about 300ml of liquid, so most people need to go to the loo about every three to four hours, says Mr Rees.
However, some conditions can lead to greater frequency. For instance, benign prostate enlargement - an age-related condition where the prostate gland blocks the bladder - increases the need to go to every hour (even so the man can only pass about 100 ml). Or it increases the time taken to empty the bladder.
“Healthy men and women generally take about 20 to 30 seconds to empty their bladder,” says Mr Rees.
Passing little and often could also be a sign of an overactive bladder, he says. Here there are faulty signals between the bladder and the brain, meaning the brain thinks it’s time to empty before the bladder is full.
Drugs, Botox and nerve- tingling devices can all help an overactive bladder.
BRAIN GROWTH: 25 years
“The first three or four years of life are what we call the critical period for the brain,” says David Price, professor of developmental neurobiology at the University of Edinburgh. ‘It’s like a telephone system where the main wiring is there, but all the fine wiring still needs connecting.’
After four years of life, the brain loses the ability to form these fine connections. By the time of puberty, the brain is pretty much fully developed, apart from one crucial part - the prefrontal cortex, just behind the forehead. This only reaches completion around the age of 25.
In 2004, a seminal study of brain scans of teenagers showed they had less brain tissue in their frontal lobes - where the prefrontal cortex is - than adults, but this increased as they got older.
PUBERTY: Three to eight years
Puberty usually starts between the ages of nine and 12 in girls, says Dr Mark Vanderpump, endocrinologist at Royal Free London NHS Foundation Trust. It then lasts between three to five years.
“The mean age of onset of breast development for girls is nine years, and the average age of periods starting is 12 - although they can start any time from eight to 20.” Completion of puberty is defined by reaching your maximum height, and girls tend to do this within two years of their periods starting, he says.
“When a girl’s periods have started, it means she has enough oestrogen in her body to fuse bones, and once our bones fuse, we can’t grow any longer.’
In boys, puberty starts between ten and 13, and lasts longer than in girls, for between five to eight years. It’s signified by an increase in testicular volume, getting taller, growing hair, and the voice deepening. But boys won’t reach their final height until their late teens, says Dr Vanderpump, with the process taking five to eight years.
A number of studies have shown children - particularly girls - are reaching puberty earlier, “probably because of genetics, nutrition improving and other environmental factors”, says Dr Vanderpump.
This may have implications for their health, with girls who start at a very young age at much higher risk of breast cancer than those who start their periods at 16, suggests Dr Marion Kavanaugh-Lynch, an oncologist and director of the California Breast Cancer Research Program in Oakland. This is because most breast cancers are fuelled by oestrogen.
Late puberty could put girls at greater risk of osteoporosis, say researchers from the Saban Research Institute at Children’s Hospital Los Angeles - oestrogen is needed for healthy bones, and puberty is a key time for bone mass development.
There seem to be fewer problems for boys hitting puberty early, but US studies suggest they might be more likely to be sexually active early and to engage in risky behaviour.
LUNG DEVELOPMENT: 18 to 22 years
Have you ever noticed your singing voice isn’t as good as it was in your youth?
Your lungs reach their peak capacity during your teens or early 20s, and decline thereafter.
“A person’s airways form in the womb, and the complete set are there by 16 weeks of gestation,” says John Henderson, professor of paediatric medicine at the University of Bristol.
“After birth, the passages get bigger, and the air sacs continue to develop, as the size of the lungs grow with the rest of the body.
“This process continues throughout childhood, and because women reach puberty earlier than men they reach peak lung function earlier, too - around their late teens. In men, it’s the early 20s.”
Certain things can stop you hitting your potential peak lung function, says Professor Henderson.
“Cigarette smoking by parents or by the mother during pregnancy, probably being born early, or being underweight at birth, recurrent lung infections or diseases such as asthma during childhood, can all make a small difference to your peak lung function.
“That means that when it declines, you’re starting from a lower level, so you’re more likely to suffer breathing problems sooner.
“It can also mean you’ll be more prone to chronic obstructive pulmonary disease.”
MENOPAUSE: Two to ten years
The median age for the onset of the menopause is 51, although the “normal” range is from 40 to 55, says Dr Vanderpump.
“I have seen the odd 55-year-old woman who is still having regular periods and it’s nothing to be worried about,” he says.
However, menopause later than this may raise the risk of breast cancer because it means the woman has had more exposure to oestrogen throughout her life. A woman having the menopause at 55 has a 30 percent higher risk than a woman at 45, according to Cancer Research UK.
A 2011 study by Imperial College London showed increasing numbers of women are hitting the menopause early - before the age of 40 - with some even starting it in their teens.
In most cases, the menopause had been triggered by surgery or chemotherapy, but for six percent of the women it was unexplained.
Early menopause carries a raised risk of osteoporosis, heart disease and stroke, because of oestrogen’s protective effect.
Once the menopause has begun, most women will experience a number of typical symptoms - such as hot flushes - for around two years, says Dr Vanderpump, although a significant proportion won’t even notice that the change has happened until they realise that their periods have stopped coming altogether.
Some women are less fortunate - a 2011 study of 400 women by the University of Pennsylvania School of Medicine found hot flushes can last ten years.
“The younger you are when you start the menopause, the more likely you are to suffer.
“This is because the oestrogen levels tend to drop more steeply in younger women.”
The menopause typically lasts for two years. One in five men over 65 will suffer the “male menopause”, or andropause, according to the European Association of Urology.
Here, diminishing levels of testosterone trigger symptoms such as fatigue, low sex drive and hot flushes. While some men say the symptoms are over within a couple of months, others report them going on for up to ten years. The condition has been linked to obesity and diabetes, and even to premature death.
However, it’s still a relatively new and controversial concept, and not easy to diagnose definitively, says Dr Vanderpump. - Daily Mail