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By Dr. Shelly Choo

When it comes to infertility, are celebrities just like us?

Model and TV host Chrissy Teigen has defended her decision to choose a baby girl while undergoing in vitro fertilization. The now pregnant celebrity has been very open about her infertility struggles and faced a Twitter backlash over her desire to have a girl with her husband, singer John Legend.

To help clear up confusion about the IVF procedure, here are answers to some common questions:

How is it possible to choose the sex of a baby?

It’s called gender selection and it’s done during in vitro fertilization (IVF). IVF is the process where an egg is removed from a woman’s body and fertilized by sperm in a laboratory to create an embryo.

Chrissy Teigen and John Legend.Jordan Strauss / Invision via AP

A few days later, cells are removed from the embryo so doctors can look at the chromosomes. The official medical term for this is “pre-implantation genetic diagnosis” or PGD, and they originally did it to help screen out serious or fatal genetic disorders. But PGD can also detect the gender of the embryo — certain markers indicate it’s a boy, or the absence of those markers tell you it’s a girl.

The next step: an embryo is implanted in the uterus. But now, some parents are first choosing the embryo based on the gender they prefer.

Here’s another high-tech gender selection method: a super-fast spinning device to separate out sperm into "male" sperm and "female" sperm. Because it’s the sperm that determines the baby’s gender.

You remember that from biology class, right?

Related: Three-parents babies are OK, experts say

Celebrities seem to love picking the baby's sex, but how common is it in the rest of the country?

While we don’t know for sure how often parents using IVF opt for gender selection, one 2008 study from Johns Hopkins found that 42 percent of the surveyed clinics offered gender selection through PGD for non-medical reasons.

And here’s an interesting fact: about 40 percent of those clinics said that they would only offer the gender selection service if it was for a second or subsequent child.

What are the main reasons parents decide not to just learn the baby's sex, but to choose it?

Some couples choose to have a baby of one sex if there’s a family history of a medical disorder linked to gender.

For example, males are far more likely to inherit genetic diseases like hemophilia or Duchenne muscular dystrophy.

And then there’s the plain old reason they want to balance out the boy/girl ratio in the family or they’ve always wanted a baby of a specific sex and they don’t want to leave it to chance and Mother Nature.

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Is this good idea or bad idea?

You can find opinions across the spectrum.

Some people believe that using gender selection non-medically is the wrong use of medical and health care resources. Critics worry about the potential gender bias in selecting one sex over the other.

Others believe this is a personal decision that parents should be allowed to make.

Related: Can infertility point to ovarian cancer risk?

Dr. Shelly Choo, a graduate of the Johns Hopkins School of Medicine, is a medical fellow with NBC News