Corinne is a few days shy of 40 weeks pregnant and so done, she says, sitting on a mat inside Yoga Tree on Valencia Street in the Mission. Today would be great. Tomorrow would be great.

Lauren (34 weeks) has a fully transverse baby lying sideways across the mothers abdomen whose butt is digging into her ribs. Jamie (37.5 weeks) has all the things. Cecilia (32 weeks) cant sleep.

As the sharing moves around the room, there are varicose veins and back pain. Sore knees and cankles and indigestion even when I havent eaten. Someone else is feeling a lot of pressure in her breasts.

Because theyre growing, says prenatal yoga instructor Jane Austin, to a burst of laughter.

How much more?

So. Much. More.

When one woman has nothing to add, Austin prods gently. Are you sure you dont want to kvetch about anything? The floor is yours.

Every prenatal class here starts with this ritual: 40 to 50 women in all phases of pregnancy propped on blankets, bolsters and dense foam blocks, introducing themselves, how far along they are and how theyre feeling. Some are still in the early weeks, their bodies not yet expanded into new dimensions and shapes. Others are in the homestretch, heaving cautiously from pose to pose, ready to expel the tiny person theyve been cultivating for the past 40 weeks. I am somewhere in the middle, potbellied but not yet waddling, listening to the stories around me as reminders of whats past and previews of whats to come.

Wherever she is in the nineish-month journey, this is the safest of places for a woman going through pregnancy. Its a room where no topic is off limits and the weirdest complaint will be met with a knowing nod and a suggestion for acupuncture, wrist guards, pregnancy tea or that couch trick.

Ive had multiple womens water break in my class, Austin says. Ive had women in labor in class.

Over the last few decades, guidelines around exercise for pregnant women have shifted significantly, from bed rest and eating for two to healthy diet and regular exercise right up until delivery. Today, an array of fitness classes caters to pregnant women and recent mothers who want to keep up with their workouts. The Mindful Body teaches pre- and postnatal yoga in Pacific Heights; the Pad has pre- and postnatal pilates in the Marina; in Cow Hollow, the Dailey Method has Dailey Baby, where moms wear their infants in frontside carriers; and the Lotus Method focuses exclusively on pre- and postnatal strength training at four locations around the Bay Area.

At Yoga Tree Valencia, near the corner of 24th Street in the Mission, prenatal yoga is scheduled six times a week; drop-in classes cost $24. On weekends, they tend to fill up, mats inching closer together and spilling into the center aisle of the room so no one is turned away.

Austin is the center of the prenatal universe at the Mission studio. Legging-clad with long brown hair and tinkling jewelry, four days a week, she inspires pilgrimages of pregnant women from around San Francisco and the larger Bay Area. For about 90 minutes at a time, Austin is yogi, therapist, confidant and comedian, a sympathetic ear and knowledgeable voice who approaches the uncomfortable truths of pregnancy and motherhood with straightforwardness, humor and practical advice.

My hips somehow feel both tight and loose, says a woman who introduces herself as Candace.

It sounds like you might be pregnant, Austin deadpans. Then she explains: The joints get mobile to prepare for childbirth while the muscles stiffen to keep everything stabilized. This class is designed to address just that kind of discomfort. Well try to balance that out.

When Austin began teaching prenatal yoga in San Francisco almost 20 years ago, she wasnt a certified yoga instructor. Vinyasa wasnt part of the vernacular, and the term athleisure was years away from being coined by someone with a strong appreciation for spandex.

Austin is a former doula and midwife who left the field to have her own children. Shed taken a prenatal yoga class during her first pregnancy, and though it wasnt quite what she was looking for, she recognized the potential. There was something there, something powerful.

She went to San Francisco studio Yoga Tree and said, This is my big refrigerator credential. This is what I love about yoga. I want to join the two.

Thats essentially what she did, developing her own prenatal curriculum through trial and error, using her experience as a midwife to inform a practice that takes a holistic view of a womans body.

Pregnancy, for many women in the U.S. is really disempowering, Austin says. CEOs, managers, independent adults in charge of their own lives: They see two lines on a First Response test and suddenly theyre treated like children, told what to eat and how to move. The idea that women are inherently unstable or inherently fragile or that pregnancy is pathology, that's the lens in which women perceive their pregnancies, culturally.

The yoga, she says, helps them step away from all the chatter of what everybodys telling them to do, and really kind of tune into their own experience. Its always my great trust and faith that women, given the space and the opportunity, that she will make good choices for her and her baby.

Cassie Sikes (35 weeks) started taking the class in her first trimester, looking for guidance on what to do and not do, how not to squash the baby.

This has made the most difference for me in my pregnancy, she says. Austin is so full of information. She really cares about whats going on. You can ask Jane anything.

Shes also a regular presence in her clients lives. While many women see their doctors once a month during pregnancy, they may go to yoga multiple times each week.

At Yoga Tree, Austin teaches prenatal classes in addition to mom-and-baby yoga (the studio floor full of tiny, wide-eyed infants), workshops where partners learn to support their pregnant companions through labor, and Mama Tree teacher trainings that have produced many of the Bay Areas prenatal yoga instructors.

Jane in particular knows how to communicate to women who are pregnant, says Yoga Tree and Yoga Works San Francisco District Manager Lidia Valdez, who attended the classes religiously during her own pregnancy and has completed the teacher training. Its more than just the yoga practice. Its a time to connect.

That doesnt mean theres any forced spirituality. I dont superimpose how I think women should feel, Austin says. Sometimes I go to (other) prenatal yoga classes, and they'll be like, Breathe love into your belly. What if you don't feel like it? What if you had a fight with your partner or youre feeling a little like, Oh god, this baby is such a parasite? But do I create conditions for women to fall in love with their babies? Yeah.

In the class I attend, after the introductions, some deep breaths and a collective ohm, Austin instructs us to stretch our arms out to the side, palms up and elbows slightly bent, eyes closed. We hold it, 50 pregnant women posed like serene Ws, breathing and listening, as Austin monologues some distraction and the muscles in our shoulders, upper backs and chests start to scream.

For those who havent done this practice before, just know that we do this for a really, really, really long time, Austin says as the burn ratchets from mild sting to a deeper fire. So if you think its already been a long time, its only been a little time.

She tells us that this is not a test, and that we can take breaks whenever we need to. She tells us to use our breath to focus our minds. When its already been an impossibly long time, she tells us that were going to hold our arms out for another two whole minutes, so we build up stamina and strength for pregnancy, for labor, for everything that comes after.

Thats what this class is all about: preparing women for motherhood. Our shoulders, at the very least, will be ready.

Sarah Feldberg is the assistant features editor at The Chronicle. Email: sarah.feldberg@sfchronicle.com.

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Preparing for birth and everything after at prenatal yoga - San Francisco Chronicle