I don’t recommend weights because I frankly don’t think pregnant people need the additional weight. Their body is changing so much from week to week that just embracing those changes and working within that framework is challenging enough. The focus too often is only on strength rather than an equal focus on balance and flexibility.
When it comes to strength training classes in pregnancy I’ve found that too many people have really bad form and posture (sometimes gets worse with lumbar lordosis) and develop hip and low back issues because they continue their same old work outs but haven’t taken into account how much more flexible their pelvis is and the additional weight from the baby. Even just in yoga I have to constantly make sure people are staying safe with their knees and back so adding weights to their routine gives me pause.
For folks who have great posture, know what they're doing, have strength trained before they got pregnant and have good body awareness, it’s a personal choice. You have to do what feels right for you.
Personally, I don’t think that using weights helps in your labor and birth, I actually think it can hinder the process which is why I don’t recommend using them prenatally.
To expand -
I like to compare birth to a marathon. When you know you’re going to run a marathon, you start building up your endurance and strength by running more. Each week you increase your distance by a little bit. It’s obviously going to be physically and emotionally challenging, but it can be a rewarding experience because it’s work that you’re doing for yourself. You can’t expect to show up to a marathon unprepared and hope that it’ll go well. A marathon lasts about 5 hours for most, labor is usually much longer. You don’t train for a marathon by doing a lot of yoga, it’s a totally different workout - so how can you start preparing for birth?
You want to do everything you can to build strength and flexibility in the muscles that need it and will be used in labor. You want to make sure that you’ve practiced and are comfortable with the exact movements that you’re going to do in labor.
From my perspective it means practicing diaphragmatic breathing, building endurance strength with squats and lunges (when you’re pregnant it's challenging enough with the changes in balance that I feel like weights aren’t necessary), building flexibility and strength in the pelvic floor with hip stretches and targeted transverse ab work that helps you stabilize and train for pushing while also allowing the belly to expand and grow. I’ve had many athletes and dancers who have have a lot of trouble finding that flexibility in letting the belly and pelvic floor expand - it’s so counter intuitive to our culture of 'sucking in’. They’ve always been taught to be strong and tight but not to release. It’s also hard to learn to let go and release in just 9 months after a lifetime of doing the opposite.
In my opinion, not all exercises are appropriate prenatally even though all are ‘safe’. The few that do make labors longer are running, pilates, barre and spin because they create more tightness in your abdomen, hips and pelvic floor. They are also more energetically about squeezing, and pulling up and tightening at a time when you’re trying to expand, be more grounded and open. Pilates and barre specifically for any asymmetric poses tend to create hip pain and pubic symphysis pain because it takes a lot more to stabilize with growing belly and an influx of relaxin. Spin and running particularly for all the work that is required just to stabilize the pelvis is fine to do prenatally but a lot of people feel tightness in their abdomen after. Spinning is great to build endurance for labor but make sure to modify, have the handle bars up much higher, sit up straight stack your shoulders over hips. If you tend to have been a dancer or have a more toned pelvic floor, it may not be the best idea for you as it may add hours to labor and birth. So if you do spin, pilates barre or strength train, make sure to balance that with equal amounts or more yoga and other exercises to help build flexibility.
Also in general cautioning for diastasis recti that weakens the abdominal muscles and risks having a mal-positioned asynclitic baby which again causes a longer labor. (It’s often those with a 6 pack before pregnancy) But all this to say that I think that a more toned pelvic floor and abdomen create longer labors and mal-positioned babies. That’s a big problem in the US because one of the major reasons for a c-section is ‘failure to progress’ so we’re not even given adequate time to let our body learn how to relax in labor.
Obviously I know that 1 pound weights in a strength training series doesn’t cause the above but I’ve had so many people come to me feeling out of sorts in their back, hips, groin after other fitness classes and then feel much better after yoga. The PTs that I’ve worked with who specialize in prenatal/postnatal also only use body weight so I feel comfortable in my stance.
The one thing that I do unequivocally endorse is swimming! It’s great because it’s no impact, builds stamina and encourages optimal positioning for baby.