My general response is that it's a 50/50 chance that a woman will have a boy or a girl. But that's not exactly true – there's actually a slight bias toward male births. The ratio of male to female births, called the sex ratio, is about 105 to 100, according to the World Health Organization (WHO).
Men inherit a tendency to have more sons or more daughters from their parents. This means that a man with many brothers is more likely to have sons, while a man with many sisters is more likely to have daughters.
“Men's sperm quality seems to deteriorate after the age of 35,” a report published in the peer-reviewed journal, New Scientist, cautions. The scientists also note that at the age of 35, the proportion of sperm carrying an X chromosome also seems to increase, meaning older dads are more likely to have daughters.
Almost everyone has around a 50% chance of having a boy and a 50% chance of having a girl. What we can say is that dad's sperm determines whether a baby will be born as a boy or a girl. About half of his sperm will make a boy and half a girl. The sex of the baby depends on which sperm gets to the egg first.
It suggests that an as-yet-undiscovered gene controls whether a man's sperm contains more X or more Y chromosomes, which affects the sex of his children. Looking at how sperm cells impact the sex of the baby is key to unraveling why some women have only boys and others have only girls.
Biological sex in healthy humans is determined by the presence of the sex chromosomes in the genetic code: two X chromosomes (XX) makes a girl, whereas an X and a Y chromosome (XY) makes a boy. In this way, it is the presence or absence of the Y chromosome in a healthy human that differentiates boy from girl.
While historically, there have been about 105 boys born for every 100 girls worldwide — which creates a “sex ratio at birth” of 1.05 — the share of boy babies has increased in recent decades. 2011 data from the World Bank show the global sex ratio at birth is now 1.07, or 107 boys born for every 100 girls.
Male births were much more likely to require the use of oxytocin, a hormone which stimulates contractions. "Of the women carrying male infants, 70 per cent of them had completely normal deliveries, compared to 76 per cent of females."
They found that the most fertile months for conceiving a boy were from September to November, while the lowest fertile period was from March to May.
Dr Michael Thomas MD (fertility specialist) actually advises that there are no methods to affect the sex of your child, including sexual position.
We might expect that naturally an equal number of boys and girls are born, but this is not the case. There are biological reasons why more boys than girls are born each year: around 105 boys per 100 girls. But in countries with a strong son preference, the sex ratio is even more skewed.
The gTWH proposes that parents who possess any heritable trait which increases the female reproductive success more than the male reproductive success are more likely to have daughters. Physical attractiveness, while advantageous for both boys and girls, is even more beneficial for girls than for boys.
She's reviewed dozens of single-child family studies and finds that only children aren't any worse off than their peers with siblings. "The studies all show that only children are not spoiled. They're no more lonely than other children, and they actually make as many friends as children with siblings," she says.
Nearly 47 percent of households with children are one-child families. The national average is 20 percent. The Pew Research Center reports that the average size of an American family has been shrinking, from 3.7 children in 1960 to 1.9 today.
All men inherit a Y chromosome from their father, which means all traits that are only found on the Y chromosome come from dad, not mom. The Supporting Evidence: Y-linked traits follow a clear paternal lineage.
At least since 1941, men have told pollsters by more than a two-to-one margin that they would rather have a boy. Women have only a slight preference for daughters. Taking all of this evidence together, the authors conclude that parents in the United States do have a preference for boys over girls.
Age and sperm
Men younger than 40 have a better chance of fathering a child than those older than 40. The quality of the sperm men produce seems to decline as they get older. Most men make millions of new sperm every day, but men older than 40 have fewer healthy sperm than younger men.
Previous studies have repeatedly found that one of the reliable predictors of the sex of the offspring is the age of the parent. Older parents are significantly more likely to have daughters than younger parents. The National Child Development Study replicates these findings from earlier studies.
Unlike nuclear DNA, which comes from both parents, mitochondrial DNA comes only from the mother.
According to a survey conducted by British parenting website Bounty, two girls are considered the best combination for parents to have a happy and harmonious family life. In their study, they surveyed 2,116 parents who had children aged 16 and under.
One study, published in 2001 in the journal Human Fetal and Neonatal Movement Patterns, found that boys may move around more in the womb than girls. The average number of leg movements was much higher in the boys compared to the girls at 20, 34 and 37 weeks, that study found.
Deliver in an upright, nonflat position.
There are a number of delivery positions that might reduce the risk of a vaginal tear during childbirth. Rather than lying down flat during delivery, deliver in an upright position. Your health care provider will help you find a comfortable and safe delivery position.
Only 7.6 percent percent of male babies and 5.5 percent of female babies were in the womb 42 weeks or longer. Males were also 1.5 times more likely to be born at 43 weeks or longer. The researchers noted that the most common reason for prolonged pregnancies is an error in calculating the due date.
It's all about Dad's genes
A man's X and a woman's X combine to become a girl, and a man's Y combines with a woman's X to become a boy. But if the sperm don't have equal Xs and Ys, or if other genetic factors are at play, it can affect the sex ratio.