The following table summarizes the changes in the carrying amount of goodwill:


(in millions)
Balance as of December 31, 2020	$	33,124

Measurement period adjustments(a)	(564)
Foreign currency translation adjustments	(211)
Balance as of March 31, 2021	$	32,349

The company performs its annual goodwill impairment assessment in the third quarter, or earlier if impairment indicators exist. As of March 31, 2021, there were no accumulated goodwill impairment losses.

The following table summarizes intangible assets:


	March 31, 2021						December 31, 2020
(in millions)	Gross carrying amount		Accumulated amortization		Net carrying amount		Gross carrying amount		Accumulated amortization		Net carrying amount
Definite-lived intangible assets
Developed product rights	$	87,602	$	(13,395)	$	74,207	$	87,707	$	(11,620)	$	76,087
License agreements	8,488		(3,129)		5,359		7,828		(2,916)		4,912
Total definite-lived intangible assets	96,090		(16,524)		79,566		95,535		(14,536)		80,999
Indefinite-lived research and development	1,727		—		1,727		1,877		—		1,877
Total intangible assets, net	$	97,817	$	(16,524)	$	81,293	$	97,412	$	(14,536)	$	82,876

The increase in definite-lived intangible assets during the three months ended March 31, 2021 was primarily due to the measurement period adjustments from the completion of the valuation of certain license agreements acquired in the Allergan acquisition. Refer to Note 4 for additional information regarding these adjustments.

Amortization expense was $2.0 billion for the three months ended March 31, 2021 and $444 million for the three months ended March 31, 2020. Amortization expense was included in cost of products sold in the condensed consolidated statements of earnings.

Indefinite-lived intangible assets represents IPR&D associated with products that have not yet received regulatory approval. The company performs its annual impairment assessment of indefinite-lived intangible assets in the third quarter, or earlier if impairment indicators exist.