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Science behind babies: The more sex you have the more likely you’ll have a boy and more girls are born in times of stress

David Spiegelhalter
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Science behind babies: The more sex you have the more likely you’ll have a boy and more girls are born in times of stress

Statistics informing fertility advice are 300 years old – before contraception
More males are born after wars and more girls are born in times of stress
More than 80 per cent of sexually active women use contraception
Despite this, nearly half of all pregnancies in the UK are unplanned

It’s the most in-depth study ever made into what really goes on in Britain’s bedrooms.
Here, in part four of the Mail’s fascinating series, a Cambridge professor explores whether fertility really falls off a cliff once a woman reaches 35, how likely it is that your contraception will fail and why more boys are born than girls.

The 300-year-old study used to work out your fertility
Everyone knows that once a woman hits 35 her fertility plummets and it’s much harder to get pregnant. Right?
Well, staggering as it sounds, this assumption is based on figures — still used in official 2013 UK fertility guidelines — that are 300 years old.

Yes, the statistics that inform modern fertility advice were gleaned from the birth patterns of pre-Revolution French peasant women.

How did this happen? French historian Louis Henry, who died in 1991, collected information from French parishes between the 18th and 19th centuries — crucially, pre-contraception — and this was used to calculate ‘natural’ fertility rates.

As a result, the fertility rate of a 35 to 39-year-old woman not using contraception is commonly assumed to be 34 per cent, compared with 48 per cent for those aged 20 to 24. From 40 to 44, it drops to 17 per cent and is less than 5 per cent by 45.
But in the 1700s many older women would have been breastfeeding from a previous birth, which would have limited their ability to get pregnant.

Additionally, those over 35 may well have already given birth to six or seven children, suffering complications that left them sterile. And, crucially, if women didn’t want more children they were probably avoiding sex altogether.

Clearly, they have little in common with modern women who have delayed starting a family by choice.

More realistic figures, based on a study of 782 couples attending Natural Family Planning Centres, show that the rates of conception in the first year, based on a woman having sex twice a week, is 82 per cent for 35 to 39-year-old women; while the chance of conceiving in the second year is 90 per cent.

For a 19 to 26-year-old, the chance of conceiving after one year are 92 per cent and 98 per cent after two.

What if you just don’t want a baby?
Nowadays, 99.9 per cent of sex does not lead to pregnancy, so there are clearly some reliable contraceptive options out there.

The 2010 Health Survey for England reveals that 82 per cent of women aged between 16 and 54 are sexually active, with 83 per cent of this group using contraception. The most popular options are condoms and the Pill — used by 22 per cent of all sexually active women. But the Pill is most popular with younger women — used by nearly half of 16 to 24-year-olds.

Implants, injections and patches — known as Long Acting Reversible Contraception — are the choice of 7 per cent of women and nearly one in five 16 to 24-year-olds.

Five per cent of women using contraceptives rely on the coil, 2 per cent on the withdrawal method and 3 per cent on ‘natural’ methods — that is, timing sex so it doesn’t coincide with a woman’s fertile period.

Abstinence might have been the main form of contraception in centuries gone by, but is used by just 0.6 per cent of women now.

Can condoms be trusted?
With more than 80 per cent of sexually active women using contraception but nearly half of all pregnancies in the UK unplanned, or where people are ambivalent, something must be going awry. So what’s the likelihood of contraception failing?

The failure rate for condoms is given by manufacturers as 2 per cent when used ‘perfectly’, a figure calculated from experiments in which volunteer couples monitored their experiences.

While many admitted having problems — from prophylactics that were put on the wrong way round to those that slipped off during love-making — in 2,248 monthly cycles where condoms were used for all sex there were just four pregnancies. From this, statisticians were able to arrive at the figure of 2 per cent.

Meanwhile, a 2002 U.S. study estimated a ‘typical’ failure rate of 18 per cent after a survey of 7,643 women — who said they relied on condoms — revealed that 14 per cent had got pregnant after a year, presumably due to incorrect use.

It was adjusted upwards to take into account abortions that women hadn’t reported. Factors which increase the failure rate are being under the age of 30, having children already, or intending to have more children, cohabiting, and being poor.

For oral contraception used properly, the risks of pregnancy are 0.3 per cent a year.

For ‘typical’ use the rate rises to 9 per cent, taking into account mix-ups on dates, forgetting to take it or stomach upsets.
Again, factors that make it more likely you’ll get pregnant are being under 30, having children, or intending to have more children, and being unmarried.

With the withdrawal method, when practised accurately the failure rate is estimated at 4 per cent, but for ‘typical use’ it soars to 22 per cent.

The Pill scare baby boom
Without doubt, the Pill revolutionised sexual behaviour — a fact illustrated by just how quickly the majority of women came to rely on it. Of those women born in 1946, 20 per cent were using the Pill in 1966 and a staggering 70 per cent in 1974.

But on October 18, 1995, the UK Committee On Safety In Medicines sent a letter to GPs warning that new evidence suggested ‘third-generation’ pills (invented in the Eighties) roughly doubled the risk of blood clots in legs.

Understandably, women were alarmed, and even though they were urged to keep taking the Pill, one GP reported 12 per cent of users stopped on the day of the announcement.

The impact was huge: conceptions in England and Wales had been falling from 1993 to 1995 but rose by 26,000 in 1996. Abortions had also been falling but rose. The result was 12,500 extra births and 13,500 extra abortions.

The young and the morning-after pill
The morning-after pill became available over pharmacy counters in 2000.

Ten years later, the Health Survey For England found this had been used by 7 per cent of sexually active women in the previous year.

But what stands out is use among the younger generation. More than one in five — 21 per cent — had taken it in the 16 to 24 age group, but only 1 per cent had used it twice.

It’s always more likely to be a boy
The figures speak for themselves — there are more boys born compared with girls.

In 2012 in England and Wales, 374,346 boys were born but only 355,328 girls, making an excess of 19,018 boys.

Put another way, five extra boys are born for every 100 girls.

This imbalance is just as well, as the male is definitely the weaker sex.

Each year an average male, no matter what age, has around a 50 per cent greater chance of dying before his next birthday than a woman of the same age.

The result is that, on average, men live fours years fewer than women.

This uneven birth rate is not a modern phenomenon.

In the 17th century, a researcher called John Graunt observed that between 1629 and 1661 139,782 boys were christened in London, but only 130,866 girls. That meant seven extra boys for every 100 girls, so the disparity was even greater then.

CASE STUDY: ‘I STOPPED OBSESSING ABOUT GETTING PREGNANT’
Rachel Wedderburn, 49, an exam invigilator, is married to Laurance, 44, a licensee. They have one daughter, Sophia, eight, and live in Walthamstow, East London.

She says:
Laurance and I met in 1998 — when I was 33 — and though I wasn’t too bothered about having children, he made it very clear before we got married that he wanted a family.

So after we married in December 2002, we started trying for a baby straight away. We were still in the first flushes of romance, so were making love twice a week.

At first, I didn’t take any notice of my fertility cycle: we were simply enjoying sex like any newly married couple. But when five months went by and I wasn’t pregnant, I obsessed about the best time to get pregnant.

I began to take ovulation tests and we’d make sure we had sex around that time. It worked. Eight months after we married, I was pregnant. But at nine weeks I miscarried.

I was upset, but also pragmatic. I was 38 and these things happen. But I also knew I was running out of time. Friends who couldn’t get pregnant were resorting to IVF and I wondered if I should do the same.

We decided to start training to be foster carers. We got really excited about the thought of helping other children if we couldn’t have our own.

As my 40th approached, I stopped obsessing about getting pregnant. I was enjoying the foster carer training and pouring my energy into organising a big birthday party for myself in Scotland.

A couple of weeks after my birthday party, I discovered I was pregnant. My period hadn’t arrived, but I never imagined it was because I was pregnant. I thought it was because I’d been partying so hard. After two weeks, though, I took a test and it was positive. We were thrilled.

Despite all the scare stories about fertility going down in your 30s, it makes sense to me that 80 per cent of 39-year-old women will get pregnant if they make love at least twice a week.

Sophia was supposed to be born in July 2006, but she was late, so came on August 2. My natural birth was fine. We thought about trying for a second baby, but when Sophia was three I was diagnosed with breast cancer and the treatment I had took away my chances of having more children. I’m fine now, but I’m so glad I invested the time to be at home with her.’

He says:  Having children was important to me — I come from a family with three brothers.

Rachel and I started trying for a baby straight after we were married and had a healthy sex life. We got slightly obsessive about making a baby, with Rachel taking ovulation tests and insisting we had to make love that day.

After the miscarriage, we became paranoid that it could happen again, but I think the secret is to make sex still enjoyable. We’d take it in turns to book weekends away and enjoy being carefree.

One thing you have to bear in mind is that when you DO get pregnant, there’s no drinking or partying for a long time afterwards, so you might as well enjoy the time you’ve got!

We were shocked but thrilled when Rachel got pregnant around her 40th birthday. We were just having a really good time up in Scotland celebrating her birthday.

Bingo, she got pregnant, and I’m convinced that part of the reason was that we were both relaxed about it.

Interviews: Jill Foster

More males are born after wars 

Examining the ratio of boys to girls born from 1838 to the modern day reveals some fascinating fluctuations.
In particular, spikes in 1919 and 1944 show that more males are born at the end of wars. This trend has been found in many other countries.

While there are those who believe a greater force must be at work ‘replacing’ the fallen with baby boys, there has been extensive research into this strange phenomenon and a number of more scientific explanations have been suggested.
One is that evolution has enabled women to have more boys in times of great loss of men as they will do better in a society short of males.

For me, the most coherent explanation is that the sex of the foetus is influenced by the hormone levels of the parents at conception, with more boys being conceived earlier on in the cycle.

The peak fertile time for a woman is around two days before ovulation but if couples have a lot of sex they are more likely to conceive before the woman reaches this peak, at an earlier point in her cycle.

Obviously during, and just after, major wars sex has to be crammed into brief periods of leave.

So a couple will maximise opportunities for making love with intense bouts of activity, with less consideration to where a woman is in her cycle.

So more conceptions are likely to occur earlier in the fertile period and therefore give a higher chance of having a boy.

This theory is backed up by another peak for boys in the UK in 1973. While there was no war on, this is when the average age of women at marriage was at its lowest — 21 — and there was a surge of teenage pregnancies.

It was a time of intense sexual activity in the young. Clearly frantic fornication produces more boys.

… and more girls in times of stress
While more boys are born at the end of wars, relatively more girls are born in times of prolonged parental stress — for example, as happened after the 1995 Japanese earthquake and in New York in the wake of the destruction of the Twin Towers in 2001.

Prolonged financial hardship also produces a similar trend.

Researchers investigating miscarriages in California found that there were more miscarriages of male babies when the unemployment rates were higher between 1989 and 2001.

So the most likely explanation for this pattern is that stress during pregnancy leads to more miscarriages of male than female foetuses.

Additionally, the flip side of the argument that more sex produces more boys is that less sex results in more girls — and stress, of course, is a well-known passion killer.

Could sneezing stop pregnancy?
Before developments in modern contraception, women would use extended breastfeeding to reduce their chances of conception, or alternatively employ internal barriers of herbs or wool soaked in honey or olive oil.

Meanwhile, the ancient Romans believed that if you sneezed after sex it would prevent conception.

While it’s doubtful many of these practices were successful, a sponge inserted in the woman before sex followed by douching afterwards has been credited with bringing down France’s birth rate in the 1700s.

Condoms were invented in 1564 by Italian surgeon Gabriele Falloppio. Originally linen sheaths held on with ribbon, they were intended as protection against syphilis, which was called the ‘French disease’, hence the English called them

‘French letters’, a name that stuck into the 20th century.

Meanwhile, the French called them ‘la capote anglaise’ — ‘the English hood’.
By the 1700s, they were made from animal’s intestines, and by the 1890s condoms came in thick rubber.

Why did pregnancy slow after 1870?
Looking at the total fertility rate — the number of babies a woman would be expected to have — for England and Wales from 1843 to 2012 something significant happens in 1870. The fertility rate starts to plummet.

It drops from nearly five births per woman down to only two births in around 1930, which is where it has remained ever since, except for spikes in 1947 and 1964, when the figure rose to around three children. Why did this happen?

One possible explanation is that improved medical provision and living conditions meant fewer babies died in childhood, so fewer ‘spares’ were needed.

Another is that it was not economically sustainable to have large numbers of children in an industrial society.

So how did women manage that?
What’s most surprising is that this trend occurred in a time before reliable, widespread contraception. So how did Victorian women manage not to get pregnant in such large numbers?

Partly, it can be explained by the fact that people married later in this period. But this wouldn’t account for such a dramatic change. Rather, it appears people were simply having less sex.

Experts of the time such as American physician William Alcott were ‘quite sure that one indulgence to each lunar month is all that the best health of the parties can possibly require’.

Couples practising abstinence and having sex just once a month were undoubtedly going to produce fewer children.

When sex reached an all-time low
If more sex produces more boys and less sex produces more girls, then looking at the proportions of boys to girls at any time may give us an idea of how much sex was being had.

Birth certificates reveal in around 1900 there were just over 103 boys born to 100 girls (compared with over 106 boys to 100 girls in 1944). This is further evidence that simply not much sex was happening at the turn of the century.

Contraception by World War 1
According to the National Birth Rate Commission’s fertility survey, in 1914 around a third of people were controlling their fertility.

Of those surveyed, 52 per cent said they used ‘continence’ (or abstinence), 13 per cent said ‘withdrawal’, 10 per cent said ‘sheaths’ (condoms) and 18 per cent said pessaries, douches and ‘other artificial means’.

PUBLISHED: 1 April 2015. By Prof David Spiegelhalter
Copyright © 1997-2014 ninemsn Pty Ltd

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