Does Missing a Session of Laser Hair Removal Mean I Have to Start From the Beginning?

Laser hair reduction treatments should be done at intervals of 4 weeks, due to the growth cycle of the hair follicle, the laser only damages the follicle when it is in its "growth" Stage.  You do not have to start treatments over necessarily but I would start spacing them at 4 week intervals for best results. 

I usually recommend an interval of 4-6 weeks.  Depending on the area, as few as 15% of hairs may be in anagen or growing phase.  Using the 4-6 week interval, there is synchronization of the growth phases of the hair follicles and more hair follicles are affected by the laser treatment.

Longer intervals than this decrease this synchronization.

If you were having your laser hair removal treatments every 12 weeks, and now it's been 16 weeks before your next one, this should not make a major difference. Regardless of the frequency of laser treatments, sometimes we see a paradoxical or 'unexpected' effect of hair stimulation (partial or significant) after laser treatment for hair reduction. I would recommend that you continue your sessions and try to schedule them every four to eight weeks maximum. Discuss with your doctor energy settings, so that the highest safe level can be used in you each time, avoiding partial regrowth of hair. No physician can predict who may get this uncommon phenomenon. Good luck! Dr. Bowes

For optimal results in the fewest number of treatments, laser hair removal should be done by a trained dermatologist or directly under their supervision, and should be done at 4 week intervals. The hair should be present but short and the best candidates are those with darker hair and lighter skin. You should not have the treatment if you have a suntan or have had recent sun exposure in the area you're planning to treat.

Lasers create a beam of highly concentrated light that penetrates deep into the skin where it delivers a controlled amount of therapeutic heat that is absorbed by the pigment located in the hair follicles. The laser is only effective when the hair follicle is in its active growth phase. The laser pulses for a fraction of a second, just long enough to destroy numerous follicles at a time and leaves the surrounding skin unaffected. Once these follicles are destroyed, the hair should not regrow even if you miss your next treatment session. It sounds as if your treatments are not as effective as they should be if you are not seeing any improvement by now.