12.	Net Loss per Share

Basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. was calculated as follows (in thousands, except share and per share amounts):


	Three Months Ended March 31,
	2018			2017
Numerator:
Net loss	$	(10,644	)	$	(6,411	)
Less: Net loss attributable to non-controlling interests		—			(535	)
Plus: Cumulative dividends on redeemable convertible preferred shares		—			(1,236	)
Plus: Accretion of bridge units and redeemable convertible preferred shares to redemption value		—			(18	)
Net loss attributable to common stockholders of Spero Therapeutics, Inc.	$	(10,644	)	$	(7,130	)

Denominator:
Weighted average common shares outstanding, basic and diluted		14,369,182			330,075

Net loss per share attributable to common stockholders of Spero Therapeutics, Inc., basic and diluted	$	(0.74	)	$	(21.60	)

The Company excluded potentially dilutive securities from the computation of diluted net loss per share as the effect would be to reduce the net loss per share. Therefore, the weighted average number of common shares outstanding used to calculate both basic and diluted net loss per share attributable to common stockholders of Spero Therapeutics, Inc. is the same. The Company excluded the following potential common shares, presented based on amounts outstanding at each period end, from the computation of diluted net loss per share attributable to common stockholders for the periods indicated because including them would have had an anti-dilutive effect:

	Three Months Ended March 31,
	2018		2017

Options to purchase common stock		2,129,082		—
Incentive units		—		422,399
		2,129,082		422,399