While the S&P/ASX 200 (Index: ^AXJO) (ASX: XJO) has enjoyed some solid gains over the past few days, it’s worth remembering that the index is still down around 2% over the past 12 months.

Despite the overall flat result, as is always the case, individual stock performances have varied from the market average.

Here are the stats on three of the best-performing stocks within the S&P/ASX 200 over the past year.

a2 Milk Company Ltd (Australia) (ASX: A2M) up 166%

Bellamy’s Australia Ltd (ASX: BAL) up 147%

Aconex Ltd (ASX: ACX) up 125%

So what was it that led to each of these shares outperforming the market so awesomely?

The share price of a2 Milk saw a huge spike occur in mid-December 2015 after the group upgraded its profit guidance for the 2016 financial year from $285 million in revenue and $22 million in earnings before interest, tax, depreciation and amortisation (EBITDA) to between $300 million and $315 million in revenues and between $33 million and $37 million in EBITDA.

The upgrade to a2’s earnings was largely thanks to better-than-expected sales of a2’s infant formula product, which as you will read below is a growth driver shared with Bellamy’s.

In August 2015, Bellamy’s (whose primary product is infant formula) released its full year results showing a 153% increase in sales to $132 million and a 617% lift in profits to $9 million.

This result provided strong momentum for the share price and was reinforced when the group released further outstanding growth figures at its half yearly results in February 2016.

Aconex is a leading provider of a collaboration platform for the global construction industry. Good news flow and half yearly results which exceeded prospectus forecasts have helped this newly-listed company’s shares rally.

Huge gains delivered by these three stocks over the past 52 weeks should not be considered an event likely to reoccur.

What’s more, despite positive momentum there is a real possibility that the rosy growth outlook for each stock is now fully captured in their respective share prices.

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Motley Fool contributor Tim McArthur has no position in any stocks mentioned. The Motley Fool Australia owns shares of A2 Milk and Bellamy's Australia. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy. This article contains general investment advice only (under AFSL 400691). Authorised by Bruce Jackson.