Beyoncé’s seemingly effortless performance at the 59th annual Grammy Awards earlier this month was all the more remarkable given that she is pregnant with twins. The inevitable march of gestation will be likely to pose even more of a challenge in April, when she is scheduled to headline Coachella.

Because pregnancy can make it harder to talk, let along belt out a tune. “It’s unfair, but it is temporary,” said Dr. Anthony Jahn, the medical director at the Metropolitan Opera.

Growing babies crowd the lungs and diaphragm, and the flood of estrogen and other hormones that comes with pregnancy can make vocal cords swell and vibrate differently. More effort is necessary, too. Expectant women have to exhale with added force to get vocal cords to vibrate and get the sound out.

So singers with a baby or two on the way often have to rethink how they breathe. Breathing deep into the abdomen is typically considered to be the most efficient way to perform, said Dr. Jahn, a Manhattan otolaryngologist at Mount Sinai West, but singers who are pregnant may have to switch to chest breathing, which is “less effective, more labor intensive and more fatiguing.”

Still, he said, “singers with Beyoncé’s talent and ability have great vocal reserves and the musical intelligence to compensate for these temporary limitations and still deliver a great performance.”

On stage at the Grammys, Beyoncé appeared to be breathing strategically. “Since her babies are under her lungs she can’t expand down, so she expands her lungs out,” on the sides, said Adrienne B. Hancock, an associate professor in the department of speech and hearing sciences at the George Washington University. “Her power and control is off the charts.”

Asked at the Grammys what it’s like to sing with twins on board, Celine Dion, who had twin boys in 2010, exclaimed, “Whoa.” Then she told reporters that when she performed while pregnant she was 65 pounds larger and scared to move. But she said of Beyoncé, “She’s a trouper. If somebody can do it, she can.”

Usually a jellylike layer enables the surface of the vocal cords, the folds of mucous membrane and muscle that stretch within the voice box, or larynx, to vibrate, said Dr. Paul E. Kwak, a laryngologist at New York University’s Langone Voice Center.

That layer, called the superficial lamina propria, is full of blood vessels that can become engorged during pregnancy, causing the surface to thicken and grow sluggish. Making sounds requires extra energy and breath.

It’s a double whammy. “The breath is shallower and the tissue is thicker, so it vibrates less easily,” Dr. Kwak said.

That reduced flexibility can make it trickier to hit high notes, so some pregnant singers avoid them altogether. But Beyoncé persisted.

A common worry is that the voice will crack, which can be embarrassing, Dr. Hancock said. Another concern is that “blood vessels are so fragile during pregnancy, you could burst one, and have a little hemorrhage.” If that occurs, complete vocal rest, including no talking, is required.

Not every singer experiences vocal changes during pregnancy. “It’s highly individual,” said Aaron Johnson, a speech-language pathologist also at N.Y.U.’s voice center.

Actually, one of Dr. Kwak’s patients, an opera singer, has not noticed any difference in vocal quality while pregnant. “She’s been able to keep her range,” he said, though she “has noticed limitations in the length of the breaths she is able to take.”

Acid reflux, a common plague in pregnancy, can also make singing painful, because stomach acid can well up and irritate the throat and drip down onto the voice box. “It’s like having heartburn in your throat,” Dr. Johnson said. “The tissue is raw, possibly swollen, making it more difficult to get the voice out.”

Hormonal changes, not just during pregnancy but during the monthly cycle and throughout the life span, further complicate the picture. The days leading up to a singer’s period can impact her voice. “The most difficult time is the week before your period, when you retain excess fluid,” said Dr. Jahn, who has worked with a variety of professional singers. “The voice may lose color and clarity, and singers often have difficulty singing softly in their higher range.”

Dr. Johnson’s lab is researching how menopause affects the voice. In menopause, estrogen levels plummet as levels of androgens, male sex hormones, rise, which can lead to swollen vocal cords. A result may be a deeper voice for some women.

Those kinds of changes can also affect a pregnant woman if she breast-feeds after giving birth. “Until you stop lactating, you usually don’t resume your periods,” Dr. Jahn said, so you “are in a sort of temporary mini-menopause from the vocal point of view.”

Thanks a lot, Mother Nature.