Among a lot of factors which influence the thickness and length of your hair, genetics is a major one. Other factors could be nutrition, exposure to pollution, stress level and lifestyle. All these factors could determine the length and health status of your hair. However, the length of a person’s hair is mostly determined by the growth phase called as anaphase.

There are different cycles for hair growth namely – anaphase, resting phase and shedding phase. Maximum hair growth happens in the anaphase where the hair bulb keeps increasing in length. After this, the hair goes into the resting phase and later it goes into the shedding phase. So during the resting phase and the shedding phase, the hair is not going to increase in length. The duration of anaphase differs in different people. In human scalp, usually it is 2 to 4 years and sometimes 4 to 6 years in some women who develop extra-long hair. So this is something that is preprogrammed into your body and is not something you can alter. But you can try to take precautions so as to avoid thyroid issues. Make sure that your body receives sufficient amount of vitamin biotin, iron, B12 and vitamin D. You can do a very simple blood test to determine whether you are taking sufficient vitamins and proteins in your diet. If you find out that the levels are low, then you go for supplementation for atleast 3 months. These will definitely help to improve the length of your hair. Another option is to use serums which improve the activity of anaphase. But you will have to use these serums for a long-term to observe satisfactory results.

As hair grows only about 1 cm per month, you will have to use the serums continuously for atleast a year to see 9 cms growth. In case of you discontinue the use of serums; the length of your hair may come back to its original length.
So if you are very keen about increasing your hair length, it is advised that you focus more on the nutrition. If you are still interested to try other options, you can go for hair extensions or continue the use of growth promoting hair serums.

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The information provided in Dr. Dixit's answer is for educational purposes only and is not intended to constitute medical advice. The information provided should not be relied upon as a substitute for consultations with a qualified health professional who may be familiar with your individual medical needs.