Net Loss per Share

3 Months Ended

12 Months Ended

Mar. 31, 2018

Dec. 31, 2017

Earnings Per Share [Abstract]

Net Loss per Share

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

15. 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):

	Year Ended December 31,
	2017			2016			2015
Numerator:
Net loss	$	(39,886	)	$	(32,641	)	$	(13,153	)
Less: Net loss attributable to non-controlling interests		(1,143	)		(7,150	)		(2,999	)
Plus: Cumulative dividends on redeemable convertible preferred shares		(6,146	)		(3,441	)		(932	)
Plus: Accretion of bridge units and redeemable convertible preferred shares to redemption value		(1,208	)		(996	)		(2,341	)
Net loss attributable to common stockholders of Spero Therapeutics, Inc.	$	(46,097	)	$	(29,928	)	$	(13,427	)
Denominator:
Weighted average common shares outstanding, basic and diluted		2,586,865			312,169			252,807
Net loss per share attributable to common stockholders of Spero Therapeutics, Inc., basic and diluted	$	(17.82	)	$	(95.87	)	$	(53.11	)

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:

	December 31,
	2017		2016		2015
Options to purchase common stock		2,011,296		—		—
Redeemable convertible preferred shares (as converted to common shares)		—		2,229,518		1,257,213
Incentive units		—		413,266		171,758
		2,011,296		2,642,784		1,428,971