Image copyright See Chin Foo Image caption In Singapore, it's very rare to see pregnant women taking part in rigorous activity outdoors

As a growing number of pregnant women are joining prenatal exercises classes, the BBC's Sarah Porter - 34 weeks into her pregnancy - attends a boot camp in Singapore.

It's 8.45am on a Saturday and Singapore's Botanic Gardens are alive with people and activity.

Local walking groups chat furiously in Mandarin, while gaggles of women push prams, coffees in hand. No-one seems particularly deterred by the rising heat.

I'm here to join a brand new exercise group called Mom In Balance. It's a franchise business founded in the Netherlands that specialises in outdoor exercise programmes for pregnant women and new mothers.

As I sit and wait for others to arrive, a group of five or six women run by me, overtaking everyone in sight. They are being led by a tall blonde woman wearing a t-shirt that says Mom in Balance. I start to panic a little.

I've done a reasonable amount of exercise throughout my pregnancy, including some swimming and a (very little) bit of running. But there is absolutely no way I'll be able to keep up with the group I've just seen sprint past.

Image copyright See Chin Foo Image caption Jantien from Mom In Balance puts Sarah Porter through her paces

Thankfully, a heavily pregnant woman decked out in running gear comes and sits next to me. I'm at the right spot, she tells me, at the right time. The 8am class I've just seen run past is for mothers getting back into shape soon after childbirth.

The tall blonde instructor returns to take the 9am class - a group which is now made up of three or four quite visibly pregnant women, together with some others.

As we set off on our warm-up, we are already dripping with sweat. As it is far from usual to see groups of pregnant women exercising outside in Singapore, passers-by stop and stare.

"Don't worry, we're famous here," one woman says to me. "Some people even stop to take photos of us."

'Powerful growth'

Despite well-documented studies that show the benefits of exercise during all stages of pregnancy, globally the prenatal exercise industry is relatively new.

In fact, while a mass of data is readily available on the $542bn (£418bn) world fitness industry, it is very difficult to find any about prenatal classes.

For example, the Global Wellness Institute (GWI) reports extensively on the fitness industry, but has no statistics whatsoever on the prenatal sector. Nor could they find any for me, from any country.

Image copyright Mom in Balance Image caption Mom In Balance's Back in Shape classes for new mothers are more vigorous

However, GWI's director of research Beth McGroarty, says the sector is definitely now expanding strongly.

"Programmes are being added at existing fitness centres and there are more prenatal yoga, Pilates and other gentle workouts available," she says.

"And given the powerful growth in prenatal fitness programmes, one can assume there will be research on this market in the future."

Founded in 2006, Mom in Balance now has franchises in 11 countries, including the US, Japan and Germany, as it tries to meet increasing demand from mums-to-be.

Image copyright See Chin Foo Image caption Others look on as Mom in Balance member Emma, 38 weeks pregnant, warms up

While the bulk of its 7,000 members are in the Netherlands, founder Esther van Diepen, is aiming to see that figure hit at least 10,000 by the end of this year, as it continues to expand around the world.

Here in Singapore, the franchise is just four months old, with 75 active members. Jantien Kroese-van den Berg, a fitness instructor and the country's new Mom in Balance franchise owner, hopes to double those numbers by the end of the year.

At 150 Singapore dollars ($108; £83) per month for a variety of classes, Jantien says she is expecting to rely more heavily on Singapore's expat community than its locals for the initial growth in numbers.

Word of mouth, she hopes, will then see more Singaporeans joining, despite some cultural opposition to pregnant women doing exercises.

'Defend themselves'

In Singapore, where the population is about 75% ethnic Chinese, together with minorities including Indians and Malays, it's very rare to see pregnant women en masse taking part in rigorous activity outdoors. Prenatal yoga and Pilates is popular, but not more vigorous exercise.

Jantien says: "There is sometimes a general feeling that you should do nothing because that might be better to hold on to your pregnancy.

Image copyright Mom in Balance Image caption Jantien wants to grow her Singapore membership numbers to 150 by the end of the year

"The Asian-born ladies in my classes - they all have to defend themselves to their families, even to their friends."

A 2015 research paper that analyses the differences in beliefs, attitudes and intentions towards prenatal exercise between women in China and Australia explains a little of what's behind this.

"In traditional Chinese culture, pregnancy is considered a vulnerable period that requires rest and recuperation, with many antenatal taboos, some of which may contrast with international guidelines on exercise in pregnancy," the report says.

"Two relevant taboos intended to avoid spontaneous miscarriage include 'not walking too fast' and 'not walking too often', which have been reported to be adhered to by the majority of Chinese women," it continues.

But Mom in Balance member Richa Nair, a Singaporean Indian, explains it's not only a traditional Chinese belief that prenatal exercise can be dangerous.

"My friends sounded a bit shocked when I described the exercises we do, but soon that turned to admiration," she says.

"With regards to my family, they are mostly horrified and believe this is a time to relax and slow down the pace of life. Their eyebrows shot into their foreheads when I told them about my prenatal exercising."

'No high impact'

Dr Ann Tan, a leading obstetrician and gynaecologist in Singapore, says attitudes towards prenatal exercise are definitely changing, though perhaps more slowly in parts of Asia.

Like most medical professionals, though, she is guarded with her advice.

"Usually I don't like any high impact in the first trimester. I like walking, you can swim too. But no high impact stuff," she says.

"The second and third trimester depends very much on the lady herself. If she's perfectly well and she's been active all her life, then she can actually resume some of her exercise, but tail it down to about 60%."

Singapore-based personal trainer Aaron Rolley, the boss of International Fitness Consultants, has worked with pregnant women for about 20 years.

Image copyright See Chin Foo Image caption A personal training session with Aaron Rolley doesn't attract quite as much as attention as the Mom in Balance groups

Charging 100 Singapore dollars for a one-on-one session, he has built a reputation as a leader in his field.

"Training during pregnancy is not about losing fat or going for a personal best," says Aaron.

"The workout for each mother will look very different, some will just stretch, foam roll and mobilise, while others will be doing chin ups and push ups. It depends on the individual and their training history."