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,
		2019			2018
Numerator:
Net loss	$	(60,925	)	$	(41,662	)

Denominator:
Weighted average common shares outstanding, basic and diluted		18,160,525			16,001,832

Net loss per share, basic and diluted	$	(3.35	)	$	(2.60	)

Per Note 19 to the Financial Statements, the Company announced a rights offering in February 2020 that settled on March 5, 2020. The rights offering contained a bonus element, whereby the exercise price at issuance was less than the fair value of the stock on the date of settlement. When a bonus element exists, the Company is required to reflect the impact of the bonus element retroactively on basic and diluted EPS for the periods ending December 31, 2019 and 2018. It was determined that the bonus element related to the rights offering in 2020 had no impact on basic or diluted EPS for the periods ending December 31, 2019 and 2018.

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:

	Year Ended December 31,
		2019	2018
Options to purchase common stock		2,969,428	2,297,810
Unvested restricted stock units		40,750	—
Series A convertible preferred stock (as converted to common shares)		1,720,000	2,220,000
Series B convertible preferred stock (as converted to common shares)		1,000,000	1,000,000
Total		5,730,178	5,517,810