When little Myla Lawrie was brought home from hospital four months ago, a veritable sea of pink greeted her, with clouds of balloons filling the sitting room, along with cards, bunting and ribbons in every shade of girliness.

For this was a very special event in the Lawrie family. When Myla was born in October, she was the first girl to be born in her family since 1809.

The last time a midwife announced ‘It’s a girl’, the Napoleonic war was still being fought, the motorcar was yet to be invented, and ‘mad’ King George III was on the English throneNo wonder her proud parents — Hannah, a 26-year-old midwife, and Mark, a 33-year-old professional golfer turned coach, from Maidstone in Kent — wanted to celebrate.

‘When Mark and I first got together, he told me how everyone in his family, for five generations, had produced boys,’ explains Hannah. ‘He warned me that the chances of us ever having a daughter were pretty remote.’

True to form, when Hannah became pregnant in 2012, no one was surprised when Mason, who’s now three, was born.

Looking back over the family tree, Hannah could see why everyone was so convinced. The last girl to be born in the Lawrie family was Mason’s great-great-great Aunt Bessie, born more than 200 years ago in the same year as Charles Darwin.

Bessie and her brother had had sons, and so the pattern had continued, all the way down to Mark and his brother Glenn. Mark has two other boys, ten-year-old Ben and seven-year-old Zac, from a previous relationship, while Glenn had one son, Reece, who’s 14.

‘I even questioned whether there could be a genetic condition that meant Mark produced only male sperm,’ says Hannah.

‘I asked numerous doctors and consultants in the hospital where I work if this was the case, but was told time and again that it wasn’t. Every time a baby is conceived, the chances of it being a girl are 50/50, they told me — but in Mark’s family, for some reason, it never was.’

While it has been long suspected that men who come from families with plenty of males have higher odds of fathering boys, and that for men with many sisters, it is vice versa, there has never been any stong scientific explanation as to why.

In a study published in the journal Evolutionary Biology, the researcher, Corry Gellatly, examined the histories of more than 900 American and European families, dating back to 1600, involving more than half a million people.

A child’s sex is always determined by the father. While women produce eggs that carry an X chromosome, male sperm cast the deciding chromosome — either an X or a Y.

Mr Gellatly found evidence that men carry a gene that determines the percentage of X and Y chromosomes in their sperm. One version of this gene produces mostly X chromosomes, another mostly Y, and a third yields equal numbers of both. But even carrying a gene that predisposes men to more sons or daughters is no guarantee of the sex of a baby — in the same way that rolling a rigged die with a six on four of its faces is no guarantee of landing a six.

Desperate to improve their chances of having a girl, Hannah came across the Shettles Method, developed by an American doctor in the Sixties, which claims the timing of conception can help determine the baby’s sex.

This is based on evidence that male sperm (Y chromosome) are faster, weaker and have a shorter lifespan than female sperm (X chromosome), which are slower but survive for longer. This means that having sex before ovulation, rather than during, should increase the likelihood of conceiving a girl, as only the female sperm will survive long enough to penetrate the egg.

Hannah used ovulation kits to work out when she was ovulating. Love-making was allowed only early on in Hannah’s cycle. As soon as the kit showed she was approaching ovulation, intercourse was banned for the rest of the month.

Hannah admits it was far from romantic, but says she wanted to give it their best shot.