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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 |

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