SAN DIEGO -- New advances in medicine now make it possible to select the gender of your baby before conception, 10News reported.

San Diego residents Phil and Lisa England want a baby girl. The couple has three boys: Tyler, 8; Trevor, 6; and Travis, 5.

"We figured we would have four kids and we always expected to have one of each," Phil said.

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"I would love to have a little girl," Lisa said.

The Englands sought help from fertility specialist Dr. Samuel Wood, Medical Director at the Reproductive Sciences Medical Center in La Jolla, Calif.

"Somewhere between 70 and 80 percent of the requests are for females," Wood said.

Most couples who request sex selection have large families already -- the result of numerous attempts to have a son or daughter, according to 10News.

"The most common time to see a couple for sex selection is after two of the same sex and then some after three," Wood said.

The sex is determined by which sperm cell ends up fertilizing the egg. Sperm cells carrying an X chromosome result in a girl and those carrying a Y chromosome result in a boy.

Wood uses two new methods to select boys or girls. The first technique is fairly simple.

"The process begins here in the lab where the sperm is separated through a special liquid filter to help enhance the odds of having a boy or a girl," Wood said.

"Only the heaviest sperm can make their way through this gradient to the very bottom of this tube," he added.

"If we want female sperm we take the upper part and remove it because that's where the male sperm is and we take the very lowest part where the female sperm is and that's what we use to inseminate," Wood explained.

The method has a 60 to 90 percent success rate for achieving a female pregnancy and between 55 percent to 75 percent success for a male pregnancy.

Wood also uses a new sperm-separation method called Microsort. It's a machine that separates male and female sperm.

"The female sperm is siphoned off in one tube and the male sperm go into another tube and that's about 90 percent effective for females," Wood said.

The Englands said that they are thankful for the options and hope it will enable them to add a feminine touch to their household.

Many fertility clinics use guidelines for family balancing, 10News reported. Some doctors will not offer it unless you have at least one child of the opposite sex from the one you want.