Nature of the Business and Basis of Presentation	9 Months Ended
Sep. 30, 2019
Organization Consolidation And Presentation Of Financial Statements [Abstract]
Nature of the Business and Basis of Presentation	1. Nature of the Business and Basis of Presentation Fulcrum Therapeutics, Inc. (the “Company” or “Fulcrum”) was incorporated in Delaware on August 18, 2015. The Company is focused on improving the lives of patients with genetically defined diseases in areas of high unmet medical need, with an initial focus on rare diseases. The Company is subject to a number of risks similar to other companies in the biotechnology industry, including, but not limited to, risks of failure of preclinical studies and clinical trials, dependence on key personnel, protection of proprietary technology, reliance on third party organizations, risks of obtaining regulatory approval for any product candidate that it may develop, development by competitors of technological innovations, compliance with government regulations, and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales. Basis of Presentation The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). The accompanying consolidated financial statements and footnotes to the financial statements have been prepared on the same basis as the most recently audited annual financial statements and, in the opinion of management, reflect all normal recurring adjustments necessary for the fair presentation of the Company’s financial position as of September 30, 2019 and the results of its operations and its cash flows for the three and nine months ended September 30, 2019 and 2018. The results for the three and nine months ended September 30, 2019 are not necessarily indicative of results to be expected for the year ending December 31, 2019, any other interim periods, or any future year or period. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2018 included in the Company’s final prospectus for its initial public offering (“IPO”) filed pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended (the “Securities Act”), with the Securities and Exchange Commission (the “SEC”) on July 18, 2019. Initial Public Offering On July 22, 2019, the Company completed an IPO of its common stock and issued and sold 4,500,000 shares of common stock at a public offering price of $16.00 per share, resulting in net proceeds of $64.0 million after deducting underwriting discounts and commissions and estimated offering expenses. Upon the closing of the IPO, all 112,500,000 shares of outstanding preferred stock automatically converted into 16,071,418 shares of common stock. In July 2019, the Company effected a one-for-seven reverse stock split of the Company’s issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each of the Company’s outstanding series of preferred stock. All share and per share amounts in the accompanying consolidated financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital. Liquidity The Company has incurred recurring losses and negative cash flows from operations since inception and has primarily funded its operations with proceeds from the IPO and issuances of convertible notes and convertible preferred stock. As of September 30, 2019, the Company had an accumulated deficit of $134.7 million. The Company expects its operating losses and negative operating cash flows to continue into the foreseeable future as it continues to expand its research and development efforts. The Company believes that its cash and cash equivalents as of September 30, 2019 will be sufficient to fund its operating expenses and capital expenditure requirements for at least twelve months from the date of filing this Quarterly Report on Form 10-Q. The Company will need additional funding to support its planned operating activities. If the Company is unable to obtain additional funding, it would be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion, or commercialization efforts, which could adversely affect its business prospects.

Nature of the Business and Basis of Presentation

9 Months Ended

Sep. 30, 2019

Organization Consolidation And Presentation Of Financial Statements [Abstract]

Nature of the Business and Basis of Presentation

1. Nature of the Business and Basis of Presentation

Fulcrum Therapeutics, Inc. (the “Company” or “Fulcrum”) was incorporated in Delaware on August 18, 2015. The Company is focused on improving the lives of patients with genetically defined diseases in areas of high unmet medical need, with an initial focus on rare diseases.

The Company is subject to a number of risks similar to other companies in the biotechnology industry, including, but not limited to, risks of failure of preclinical studies and clinical trials, dependence on key personnel, protection of proprietary technology, reliance on third party organizations, risks of obtaining regulatory approval for any product candidate that it may develop, development by competitors of technological innovations, compliance with government regulations, and the need to obtain additional financing. Product candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance-reporting capabilities. Even if the Company’s development efforts are successful, it is uncertain when, if ever, the Company will realize significant revenue from product sales.

Basis of Presentation

The accompanying consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States of America (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative GAAP as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”).

The accompanying consolidated financial statements and footnotes to the financial statements have been prepared on the same basis as the most recently audited annual financial statements and, in the opinion of management, reflect all normal recurring adjustments necessary for the fair presentation of the Company’s financial position as of September 30, 2019 and the results of its operations and its cash flows for the three and nine months ended September 30, 2019 and 2018. The results for the three and nine months ended September 30, 2019 are not necessarily indicative of results to be expected for the year ending December 31, 2019, any other interim periods, or any future year or period. These consolidated financial statements should be read in conjunction with the Company’s audited consolidated financial statements and the notes thereto for the year ended December 31, 2018 included in the Company’s final prospectus for its initial public offering (“IPO”) filed pursuant to Rule 424(b)(4) under the Securities Act of 1933, as amended (the “Securities Act”), with the Securities and Exchange Commission (the “SEC”) on July 18, 2019.

Initial Public Offering

On July 22, 2019, the Company completed an IPO of its common stock and issued and sold 4,500,000 shares of common stock at a public offering price of $16.00 per share, resulting in net proceeds of $64.0 million after deducting underwriting discounts and commissions and estimated offering expenses. Upon the closing of the IPO, all 112,500,000 shares of outstanding preferred stock automatically converted into 16,071,418 shares of common stock.

In July 2019, the Company effected a one-for-seven reverse stock split of the Company’s issued and outstanding shares of common stock and a proportional adjustment to the existing conversion ratios for each of the Company’s outstanding series of preferred stock. All share and per share amounts in the accompanying consolidated financial statements and notes thereto have been retroactively adjusted for all periods presented to give effect to this reverse stock split, including reclassifying an amount equal to the reduction in par value of common stock to additional paid-in capital.

Liquidity

The Company has incurred recurring losses and negative cash flows from operations since inception and has primarily funded its operations with proceeds from the IPO and issuances of convertible notes and convertible preferred stock. As of September 30, 2019, the Company had an accumulated deficit of $134.7 million. The Company expects its operating losses and negative operating cash flows to continue into the foreseeable future as it continues to expand its research and development efforts.

The Company believes that its cash and cash equivalents as of September 30, 2019 will be sufficient to fund its operating expenses and capital expenditure requirements for at least twelve months from the date of filing this Quarterly Report on Form 10-Q. The Company will need additional funding to support its planned operating activities. If the Company is unable to obtain additional funding, it would be forced to delay, reduce or eliminate some or all of its research and development programs, product portfolio expansion, or commercialization efforts, which could adversely affect its business prospects.