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When a cat's in heat, she advertises the fact to increase her chances of finding a boyfriend and having kittens. One way she does this is by vocalizing incredibly loudly and constantly. To an inexperienced owner, it may seem like she's in pain, but this is perfectly normal behavior. You can either choose to deal with the extra noise, or try to calm her down.  Try plugging in a Feliway diffuser before your cat comes into heat. This synthetic feline pheromone surrounds her with safe, familiar scents. Pheromones are chemical messengers — a similar idea to the pheromones a female gives off to advertise she is in heat. But the ones in Feliway might have a calming, soothing effect on her. It doesn't work immediately, but the pheromones build up the cat's sense of well-being over a couple of weeks. As such, it's best to plug the diffuser in early and leave it running constantly in the background, so she gets the benefit by the time she comes into heat. Not all female cats do this, but some mark their territory with urine. Urine has potent scent markers that she uses to attract mates. Again, there's nothing you can do to stop this behavior without getting her spayed, so you just have to make do. A few things you can do to make life a little easier include:  Making sure she has a clean litter box at all times. Hopefully her house training will lead her to use the box instead of marking in the house. Deodorize immediately if she does mark. This will discourage her from returning to that spot to mark it again. Use an "enzymatic" cleaner to completely remove the urine odor. The enzymes in these products break cat urine down better than any other cleaner. Allow the product to air dry for best results. A home-made alternative is to dilute some biological washing powder in water. Use this to clean the area, then rinse it with water.  Clean again with a solution of bicarbonate of soda. Finish by rinsing one last time. Always test cleaning products on a small, hidden patch of fabric before treating a marked patch for odor. Your cat goes on a massive hormonal roller coaster,which changes her behavior when she's in heat. One difference is that her body language and social behavior might change quite a bit.  She might seek out company far more than usual. She may demand that you rub her lower back. When you do, she might flirtatiously hold her tail to one side to better expose her genital area. She may 'commando crawl' across the floor by flattening her front half against the floor, then crawling with her rear end pushed up in the air. She is also likely to squirm and roll around enthusiastically. When combined with the loud crying, it might look like she's in pain. Don't worry — she's just flirting. There's nothing to be done about this behavior. It's perfectly normal, and as long as no males are able to reach her, she'll be fine. Tire her out with regular play sessions. This may lessen her energy and help her to settle down and sleep instead of writhing around and yowling. Some female cats enjoy an extra fuss or even a massage when they are in heat. But don't be surprised if she starts acting flirty with you and presents her backside! Many female cats don't eat properly when they are in heat. This can cause them to lose weight and body condition, which worries their loving owners. But don't try to make up for her lack of appetite with higher calorie cat food. This will only supply her with even more energy to cry and call. Instead, leave large amounts of her regular cat food available for free-feeding. She can snack whenever she feels like it. If you own more than one cat, think of how stressful it is for the others to have a calling female in their midst for one week out of every three. This can be unsettling for them. The Feliway diffuser you use to calm the behavior of the cat in heat will also make your other cats feel better. Again, Feliway takes a couple of weeks to build up in your cats' systems, so plug in the diffuser before your female goes into heat.
Calm her loud meowing. Deal with scent marking. Be prepared for clingy behavior. Give her extra attention. Don't change her feeding routine. Decrease the stress on your other cats.