Hey mum! What you're experiencing sounds like melasma, it's super common in pregnancy, caused by those hormonal changes ramping up your melanin production. The good news is it often fades after delivery, though for some mums it takes a few months. Someone in my mum group mentioned hers improved significantly once she was out of the postpartum phase, so it's usually not permanent even if it feels scary right now.
For now, sunscreen really is your best friend, a high SPF daily will help slow it down and stop it from getting darker. Some mums swear by mineral or physical sunscreens rather than chemical ones during pregnancy, just to be extra safe. The yellowish tone should even out once hormones settle, but it's definitely worth mentioning to your gynae at your next visit so they can confirm it's just melasma and rule out anything else. Take care!