** Shares of Twilio TWLO.N up 4.2% to $105.06 before the bell as cloud communications platform set to join S&P MidCap 400 index .IDX next week

** TWLO to replace Amedisys AMED.O in IDX effective Tues, Aug 19, S&P Dow Jones said late Thurs

** Insurer UnitedHealth UNH.N acquired AMED in $3.3 bln deal completed on Thurs

** Stephens analyst Melissa Roberts said she anticipates passive funds will need to purchase 19.8 mln TWLO shares, which implies 7 days of buying demand based on stock's 3-mth avg daily trading volume of 2.9 mln shares

** San Francisco-based TWLO has ~$15.5 bln market cap based on ~153.4 mln shares outstanding

** On Aug 8, TWLO shares dropped 19% after co posted Q2 beat but its Q3 guidance failed to impress

** Through Thurs close, shares down ~7% YTD and off ~34% from 1-yr-high hit on Jan 31

* Of 28 brokerages covering TWLO, recommendation breakdown is 19 rate "strong buy" or "buy", 7 "hold" and 2 "sell"; median PT is $130, per LSEG data

(Lance Tupper is a Reuters market analyst. The views expressed are his own)

((lance.tupper@tr.com 1-646-279-6380))