Exhibit 5.1

One Financial Center Boston, MA  02111 617 542 6000 mintz.com

November 1, 2018

OvaScience, Inc.

9 Fourth Avenue

Waltham, MA 02451

Ladies and Gentlemen:

We have acted as counsel to OvaScience, Inc. (the “Company”) in connection with the filing by the Company of a Registration Statement on Form S-4 (as amended, the “Registration Statement”) with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”). The Registration Statement provides for the registration by the Company of up to [17,722,199] shares of its common stock, par value $0.001 (the “Shares”) upon the consummation of the merger (the “Merger”) of Orion Merger Sub, Inc., a Delaware corporation and a direct, wholly-owned subsidiary of the Company (“Orion Merger Sub”), with and into Millendo Therapeutics, Inc., a Delaware corporation (“Millendo”), pursuant to the Agreement and Plan of Merger and Reorganization, dated August 8, 2018, as amended on September 25, 2018 and on November 1, 2018, by and among the Company, Orion Merger Sub and Millendo (the “Merger Agreement”).

In connection with this opinion, we have examined the actions taken by the Company in connection with the authorization of the issuance of the Shares, and such documents as we have deemed necessary for the purpose of rendering this opinion. In our examination, we have assumed the genuineness of all signatures, the legal capacity of natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as certified or photostatic copies and the authenticity of the originals of such copies. As to questions of fact material to this opinion, we have relied upon certificates or comparable documents of public officials and of officers and representatives of the Company.

Our opinion is limited to the federal laws of the United States and the General Corporation Law of the State of Delaware and we express no opinion with respect to the laws of any other jurisdiction. No opinion is expressed herein with respect to the qualification of the Shares under the securities or blue sky laws of any state or any foreign jurisdiction.

Please note that we are opining only as to the matters expressly set forth herein, and no opinion should be inferred as to any other matters. This opinion is based upon currently existing statutes, rules, regulations and judicial decisions, and we disclaim any obligation to advise you of any change in any of these sources of law or subsequent legal or factual developments which might affect any matters or opinions set forth herein.

Based upon and subject to the foregoing, it is our opinion that, when the Shares are issued and delivered by the Company in accordance with the Merger Agreement, the Shares will be validly issued, fully paid and non-assessable.

BOSTON      LONDON      LOS ANGELES      NEW YORK      SAN DIEGO      SAN FRANCISCO     WASHINGTON

MINTZ, LEVIN, COHN, FERRIS, GLOVSKY AND POPEO, P.C.

MINTZ
November 1, 2018
Page 2

We understand that you wish to file this opinion with the Commission as an exhibit to the Registration Statement in accordance with the requirements of Item 601(b)(5) of Regulation S-K promulgated under the Securities Act and to reference the firm’s name under the caption “Legal Matters” in the prospectus which forms part of the Registration Statement, and we hereby consent thereto. In giving this consent, we do not admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations of the Commission promulgated thereunder.

Very truly yours,

/s/ Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.

Mintz, Levin, Cohn, Ferris, Glovsky and Popeo, P.C.