Soph, if you're losing strands at the base there's not a lot you can do, aside from being careful not to inadvertently pull them out. Pregnancy messes with your growth cycles and it's likely those strands' growth cycles were ended prematurely. Here is some advice for protecting length.
If you're losing length down the shaft you can use the same advice as long hairs. I am one. I've had hair to my butt. Currently down to the small of my back, when I cut it to bra strap length a year and a half ago for my wedding. My hair grows FAST! (It's also getting annoying and I'll probably cut it back to bra strap soon.
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Wash your hair less often. (If you wash less often your hair will start taking longer to get greasy. Your body will learn not to overproduce nourishing sebum on your scalp. Washing also dries it out and makes it brittle. I tend to pin it up so I can wash the rest of me more often.) Try to find ways around brushing it roughly with a hair brush, like braiding while you sleep and not roughly handling it coming out of the shower. Use fewer elastic bands as these weaken the shaft around the tie point. (I used to bun my hair using spin pins, look them up) Comb instead of brushing when possible. (I actually lack the patience for this) Do not brush when wet if you can help it. Don't use heat if you can help it, though blow drying before brushing is often better overall. Your hair is more likely to stretch and snap when wet. Also if you're like me, it clings to itself and just retangles. You can carefully comb when wet.
Also, trim less often. Those people who say trimming speeds growth? Liars. Your hair grows no faster because you trimmed it. You're better off preventing split ends and trimming less often.
Some people react better to different hair products too. You might consider finding conditioner that omits silicons for example, as it can leave buildup. Other people find the silicons do them good.
As a side note the length of your growth cycle (which indicates terminal length for your hair) is largely genetic, though as I said pregnancy hormones can sometimes lengthen these or cut them short. I believe the average growth cycle for human head hair is between 2 and 6 years. (and the rest of your body a couple months.
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