A top expert on reproductive ageing has said that women need to face up to the facts of biology and have babies before the age of 35.
For a lot of women who haven't had children either because they haven't met the right person yet or because they are trying to juggle their career and relationship, this will certainly come as patronising advice.
Professor Mary Herbert argues that 35 is when the "clock strikes 12" and the chances of motherhood are cut by chromosomal damage no amount of fertility treatment can reverse.
Too many women fail to understand the hard reality that their reproductive time runs out relatively early in life, she maintains.
Talking to HuffPost UK Lifestyle, Dr Raj Persaud who practises at Harley Street says: "In my opinion this panel of biological scientists don't seem to have properly understood why women postpone having babies until later, and why this is an increasing trend - the scientists mention high childcare costs and careers - making the women who are postponing childbirth into their late thirties as seeming somewhat selfish. In fact in my experience the real cause is women are not confident they have met the right partner with which to bring children into the world, or they are single because the right partner has yet to arrive.
"These comments on fertility are likely to create more panic and may lead to a proportion of women into having babies with the wrong man in a hasty attempt to get pregnant before 35. Then if there is separation or divorce or acrimony in the relationship - none of this is good for mother or child. The scientists should have weighed up the psychological risks involved in raising anxiety as well as quite correctly alerting women as to the biological hazards of postponing childbirth."
Author Joanne Mallon added: "People don’t want to hear that their most fertile age is in their twenties as many simply don’t feel ready to even think about starting a family then. And we can’t always plan our lives to the extent that this research suggests, even if we might like to. Relationships, career, health, just plain old life – all have a part to play, and any of these factors could mean that you end up having a baby earlier or later than you intended to, or not at all."
The professor, from the Institute for Ageing and Health at the University of Newcastle, spelled out her message at the British Science Festival.
Explaining the chromosomal abnormalities that appear in an older woman's eggs, she said: "What we can say for sure is that reproductive technologies do not do much to buy time.
"Perhaps the most important message to give is that the best cure of all is to have your babies before this clock strikes 12.
"I would be getting worried about my daughter if she hadn't had a child by 35."
In response to the remarks by Professor Herbert, HuffPost UK blogger Sarah Lothian says: "I started a family at 36, and have three children. If only life was that simple. There are many reasons why women don't, or can't start a family in their 20s or early 30s. Most women are already all too aware of the risks."
She and other experts speaking at the festival, held at the University of Newcastle, stressed that family planning should involve thinking about the timing of pregnancy as well as contraception.
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Women are born with a pool of one to two million eggs that decrease in number until at the menopause they effectively run out.
But it is not only the number but the quality of eggs that is reduced by the passing years.
Over time, a woman's eggs can end up with too many or too few chromosomes - the strands of packaged DNA that contain our genes.
The chromosomal abnormalities lead to infertility, stillbirth, or birth defects such as Down's syndrome.
Sadly, there is no practical solution to this problem, the experts point out.
Even freezing eggs for future use, a technique still in the early stages of development, could not guarantee success and was prohibitively expensive.
Story continues below the slideshow
The common household chemical bisphenol A (BPA) can lower sperm counts and motility, according to<a href="http://www.rodale.com/bpa-and-unexplained-infertility"> a study</a> in Reproductive Toxicology. Food packaging is a major source of BPA, which can seep into the food. Most food cans are <a href="http://www.msnbc.msn.com/id/41025571/ns/health-sexual_health/t/shooting-blanks-surprising-sperm-killers/#.ULaH2WmMF9R">coated with BPA resin</a>, and acidic canned foods are especially risky.
Certain plastics, including vinyl used in some sex toys, release phthalates — plastic-softening chemicals that have been <a href="http://news.harvard.edu/gazette/2003/05.22/18-semen.html">tied to lower reduced sperm count and quality</a>. Glass, silicone and green sex toys are a safer bet.
Phthalates are found in many shampoos, soaps, deodorants and shaving creams. Look for natural, phthlate-free products instead.
It's not surprising that smoking cigarettes is bad for sperm, but smoking weed isn't much better. Heavy marijuana smokers have been found to have <a href="http://news.bbc.co.uk/2/hi/health/3186686.stm">significantly lower sperm counts and sperm with bad swimming strategies</a>.
Stress can cut sperm count dramatically by <a href="http://scienceblog.com/community/older/1997/B/199701021.html">inhibiting the production of testosterone</a>.
Women aren't the only ones who should slow down on drinking while trying to conceive. Alcohol can <a href="http://menshealth.about.com/cs/stds/a/healthy_sperm.htm">inhibit sperm production</a> and lower sperm counts.
BPA is found on about 40 percent of cash register receipts, according to <a href="http://www.sccma-mcms.org/full+article/bisphenol+a+in+cash+register+receipts/">a 2010 University of Missouri study</a>. The chemical can penetrate skin or be passed from hand to mouth to the digestive track. Skip the receipt or wash your hands after signing.
Isoflavones found in soy products <a href="http://www.isoflavones.info/">mimic estrogen</a>, and a <a href="http://www.washingtonpost.com/wp-dyn/content/article/2008/07/25/AR2008072502985.html">2008 study</a> linked soy consumption to significantly lower sperm counts. The soy food industry <a href="http://www.foxnews.com/story/0,2933,303160,00.html">disputed the study's findings</a>.
Polychlorinated biphenyls (PCBs) are a group of toxic chemicals that have been banned but have accumulated in certain fish — especially predator fish and bottom-feeders. They are associated with <a href="http://www.epa.gov/teach/chem_summ/PCB_summary100809.pdf">decreased sperm counts and integrity</a>. Smaller, younger fish <a href="http://www.dhs.wisconsin.gov/eh/hlthhaz/fs/pcblink.htm#Which fish are safer to eat?">are considered safer</a>.
Chemicals called perfluoroalkyl acids are used in the manufacturing of nonstick products such as Teflon, Gore-Tex and wax paper and can significantly lower sperm counts, according to <a href="http://www.rodale.com/male-infertility-and-nonstick-chemicals">a Danish study</a>.
Of course not having sex at all won't help, but some couples abstain for days or weeks in hopes of saving sperm until the woman is ovulating. This doesn't work. Researchers have found that while it does take a day or two for sperm to replenish themselves after ejaculation, waiting any longer than that <a href="http://www.guardian.co.uk/uk/2003/jun/30/highereducation.sciencenews">can cause them to lose motility and change shape</a>.
Yet women were increasingly delaying motherhood. Between 1986 and 2008 the typical age at which a British woman had children rose from about 27 to 29. Over the same time period, the proportion of mothers aged 35 to 39 rose from 6.8% to 17%.
"Women tell me it's their career," said Prof Herbert. "In a sense I think that's misguided, because there's no career where it gets less busy as you go on."
She thought it was something society needed to address as well as individual women, who were not being properly informed.
"I think it goes beyond the message to women," she said. "It goes to the message to policy makers. We need to understand what the barriers are to women having their children early."
Colleague Professor Judith Rankin, from the Institute of Health and Society at the University of Newcastle, said: "The big message from where I'm coming from is I don't think women understand the risks there may possibly be when they reach older ages.
"There is quite good evidence about some of these outcomes and for me the whole driver for ante-natal care in this country is about informed choice.
"How can women make an informed choice if they're not being told the risks and told that side of the story?"
HuffPost UK blogger Georgy James