Chrissy Teigen and John Legend knew the sex of their second child months ago — here's how

Elaine Thompson/AP

• Chrissy Teigen and John Legend announced that they're expecting their second child. 
• They already know that it's a boy because they used in vitro fertilization, or IVF.
• In IVF, genetic tests can determine the sex of an embryo before it's implanted in the mother.
• Teigen hinted at this news in January 2017. 

Chrissy Teigen and John Legend are expecting their second child. The couple announced the pregnancy in an Instagram video featuring their daughter, Luna, who's 19 months old. 

Teigen also showed off her growing baby bump in a Snapchat photo. "Very excited not to have to hide this anymore," she captioned it. 

Chrissy Teigen/Snapchat

When the due date arrives, however, Teigen and Legend  won't be surprised by the baby's sex. In fact, they've known that they're having a boy for quite some time: before Teigen became pregnant. Back in January, Teigen revealed on Twitter that the couple's only available embryo — created using in vitro fertilization, or IVF — was male. It appears that boy was the only possible outcome.

With IVF, parents really can learn the sex of their baby ahead of time.

It's not uncommon for parents using IVF to determine the sex of an embryo before it's implanted, Health reports. This is possible thanks to a test called preimplantation genetic diagnosis (PGD).

First, a quick refresher on the basic steps of IVF: Several eggs are extracted from the mother's ovaries, then manually combined with the father's sperm in a laboratory. Next, the fertilized eggs grow into embryos.

That's when parents can opt for PDG — a procedure that analyzes the embryos to make sure that they're genetically normal. This can costs anywhere from a few hundred to a few thousand dollars, Health reports.

Flickr/ZEISS Microscopy

The American Pregnancy Association notes that this extra screening can benefit parents who risk passing down a genetic disorder to their kids. It's sometimes recommended for older mothers and women who've had recurring miscarriages, too, according to Penn Medicine. 

Determining whether the embryo is male or female isn't the point of PDG, but the sex of embryos is revealed as a byproduct of all that testing, Health reports. In some cases, parents might be told which of their healthy embryos are male and which are female.   

"If more than one embryo is normal, and there happen to be both males and females, we don't impose on patients which one they should use," Dr. Avner Hershlag told Health. "If all other things are equal, it becomes their personal choice."

But "picking" the sex of your baby is considered controversial.

Charley Gallay/Getty Images Entertainment

Back in 2016, while pregnant with Luna, Teigen revealed that she had specifically chosen to have a girl. 

"Not only am I having a girl, but I picked the girl from her little embryo. I picked her and was like, 'Let's put in the girl,'" she told People magazine. "I think I was most excited and allured by the fact that John would be the best father to a little girl."

This admission stirred controversy online, the New York Times reported, since some people believe that selecting embryos based on their sex is unethical and sexist. Others take issue with the potential "throwing away" of embryos that aren't the desired sex. 

Teigen eventually responded to the backlash on Twitter, writing, "We didn't create a little girl. we had multiple embryos. girls and boys. we simply chose to put in a female first … you'd be surprised at how many people you know go through this. also every doctor knows the sex of the embryos, it isn't some grand secret."

Vogue reports that some doctors take issue with parents who use IVF specifically to choose a baby's sex. The American College of Obstetricians and Gynecologists also opposes "all forms of sex selection" that aren't related to a sex-lined genetic condition. But what if the couple chose IVF for different reasons, like infertility, and simply found out the sex during the process? It's a complex debate.

For her second pregnancy, Teigen said she and Legend's only remaining embryo was male.

During a red carpet interview in January 2017, Teigen was asked about having more children and said that "a little boy" would be "next for sure."  The quote got a lot of attention, prompting her to post a clarifying tweet. "Since this is coming up again, I said our next baby would be a boy because that is the embryo we have left," she wrote. "A boy. So. Yeah." (For reference: Couples sometimes choose to freeze extra IVF embryos for future use, according to Columbia University Medical Center.)

It's not clear whether Teigen and Legend chose to save only a male embryo, or whether they had one healthy embryo that just happened to be male. 

Either way, fans of the couple are clearly excited about the new addition to the family. As one commenter wrote on Teigen's Instagram: "Another phenomenally gorgeous baby [to] look at, I can't wait!"

INSIDER reached out to Teigen's representative for comment. 

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