Steps

1. Never assume that a girl is in this situation unless you are completely sure, as it is incredibly offensive. Although some men may be sympathetic at 'that time of the month', the general perception that women have is that men can't actually understand what a woman goes through during menstruation, given that they do not experience it. To assume that a girl is having PMS, without being told, is arrogant, unreasonable, invasive, and insulting.
2. Get informed about how a woman's body, ovulation and menstruation cycle, and hormones function and fluctuate. If you don't know the exact reasons for its existence, you may never understand why her emotional shifts occur. This will leave you feeling like your girlfriend is treating you like a doormat for no or close to no reason and might take a toll on your relationship.
3. Realize that you cannot control the situation no matter what you do. Realize also that mood swings in girls in this situation are not necessarily under their control. Knowing this, and understanding that it will happen one way or another is your best strategy.
4. Find out what her usual pattern is, i.e. is she usually in pain/depressed/tired? Is her libido strong or practically non-existent? Try to adapt to these patterns. If her libido lessens during that time, don't act pushy. It will depress or anger her that she is disappointing you. If she has a strong libido, don't stay away from her just because it's "that time of the month". Figure something out that does not make either of you feel rejected or obligated.
5. Don't antagonize her. Easier said than done, but avoid piquing her temper as much as possible. It's also better to avoid asking too many questions. Questions are normal, but don't overwhelm her with pointless questions. She may feel pushed or untrusted. Save these for after the menstruation cycle.
6. Do something thoughtful for her. If possible avoid getting something in return: this will prove to her that you want her to be happy, and it's not that you want something from her.
7. Prepare for extreme reactions, in both directions. If you're nice to her to make her feel good, she may cry tears of joy over some flowers or you doing her dishes. Don't let it freak you out. Hug her (carefully, this is not the time for rib-cracking squeezes), give her a kiss and hand her a tissue. She'll love you for it.
8. Comfort yourself with an understanding that she is not enjoying her period and treat her with tenderness, but with a quiet strength. Your girlfriend will react well to your self assurance.

Tips

• Don't be afraid of your girlfriends menstrual cycle. really, it's just a time of the month that you can use to make her love you more. Boyfriends who run the other way everytime a girl is on her period comes across as unsympathetic and immature. Be sweet during this time of the month; the relationship will form a stronger bond because of it.
• Be forgiving and reassuring. Her insecurities are likely to come up around sharing this delicate point in her cycle with you, and she's probably already worried about offending you or letting you down. Besides that, when someone's in pain, it's easy to stick their foot in their mouth and spew that pain onto others, and later regret it. Understand that she's not trying to hurt or offend you; be quick to see the innocence behind her actions. She may, however, be asking for space; be sure to listen for that message and check in. She may just need to be alone.
• Be strong but not dominating.
• Be gentle when touching; she may be sore/have a headache/ have cramps.
• LISTEN to what she is telling you and understand that it is very important to HER even if you think it is ridiculous.
• Flowers, her favorite dinner, a heating pad, massage! If she's very sore, bubble bath and a chick flick.
• If your wife/girlfriend is especially sensitive to PMS, it pays to keep track of her periods.
• Remember, some women suffer more from PRE-menstrual symptoms, which are typically emotional fluctuations caused by the hormonal cycle, whereas some are more affected by the direct physical pain of menstruation (which is not PMS).
• Don't assume anything and help out.
• Women tend to think and feel in different ways than men do. When it comes to their periods, sensitivity can be much more extreme than usual. Be careful and treat her well.
• If you know her cycle and one day she's just really emotional when it isn't that time of the month, remember there could be other reasons. On rare occasions, women may experience cramps at other times of the month (such as ovulation cramps, or implantation cramps if she gets pregnant). It may also be that she is going through another kind of physical pain, but more likely it is something else altogether. Take a breath, set time aside for her without compromising your own needs too far, and be there to share her joy and sorrow. Don't assume anything and don't be too quick to take anything personally.

Warnings

• Stupidity can destroy a relationship.
• Even though she may be mean to you, DO NOT tolerate her physically abusing you. Maybe a slap is tolerable but unprovoked violence during any time of the month is abuse.
• Not all women have PMS symptoms. Don't try to blame your relationship issues on your girlfriend's period. This is just immature. If she really suffers from PMS, that's just a few days out of a whole month. Dig deeper to find the real core of your problems.
• Probably the worst mistake you could make is assuming that she's PMSing when she really isn't, because it shows that you don't respect her and you think she's irrational.
• If she IS going through PMS, NEVER EVER mention to her that her current anemic state is due to PMS. Especially if she is having an argument/fight with you.
• Do not share the fact that she is on her period with anyone. This is very private information and whoever you share it with may talk to your girlfriend about it. This will make her very uncomfortable and she may not feel like she can confide in you.

Things You'll Need

• Patience. AMAZING amounts of it.
• Heating Pads
• A kind heart