UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
WASHINGTON, D.C. 20549

SCHEDULE 13G

UNDER THE SECURITIES EXCHANGE ACT OF 1934
ANNUAL FILING


AMAG PHARMACEUTICALS INC
(NAME OF ISSUER)
COMMON STOCK
(TITLE OF CLASS OF SECURITIES)
00163U106
(CUSIP NUMBER)
12/31/2017
(DATE OF EVENT WHICH REQUIRES FILING OF THIS STATEMENT)

CHECK THE APPROPRIATE BOX TO DESIGNATE THE RULE PURSUANT TO WHICH THIS
SCHEDULE IS FILED:

                              (X)  RULE 13D-1 (B)
                              ( )  RULE 13D-1 (C)
                              ( )  RULE 13D-1 (D)

*THE REMAINDER OF THIS COVER PAGE SHALL BE FILLED OUT FOR A
REPORTING PERSON`S INITIAL FILING ON THIS FORM WITH RESPECT TO THE
SUBJECT CLASS OF SECURITIES, AND FOR ANY SUBSEQUENT AMENDMENT
CONTAINING INFORMATION WHICH WOULD ALTER THE DISCLOSURES PROVIDED
IN A PRIOR COVER PAGE.

THE INFORMATION REQUIRED IN THE REMAINDER OF THIS COVER PAGE SHALL
NOT BE DEEMED TO BE "FILED" FOR THE PURPOSE OF SECTION 18 OF THE
SECURITIES EXCHANGE ACT OF 1934 ("ACT") OR OTHERWISE SUBJECT TO THE
LIABILITIES OF THAT SECTION OF THE ACT BUT SHALL BE SUBJECT TO ALL
OTHER PROVISIONS OF THE ACT (HOWEVER, SEE THE NOTES).












CUSIP NO: 00163U106   13G         Page 2 of 7 Pages

1.  NAME OF REPORTING PERSON: STATE STREET CORPORATION
    I.R.S. IDENTIFICATION NO. OF THE ABOVE PERSON: 04-2456637

2.  CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP

    NOT APPLICABLE

3.  SEC USE ONLY

4.  CITIZENSHIP OR PLACE OF ORGANIZATION

    BOSTON, MASSACHUSETTS

5.  SOLE VOTING POWER

    0 SHARES

6.  SHARED VOTING POWER

    2,963,701

7.  SOLE DISPOSITIVE POWER

    0 SHARES

8.  SHARED DISPOSITIVE POWER

    2,963,701

9.  AGGREGATED AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

    2,963,701

10. CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES

    NOT APPLICABLE

11. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

    8.38%

12. TYPE OF REPORTING PERSON

    HC


CUSIP NO: 00163U106   13G         Page 3 of 7 Pages


1.  NAME OF REPORTING PERSON:
     SSGA FUNDS MANAGEMENT, INC.
     I.R.S. IDENTIFICATION NO. OF THE ABOVE PERSON: 04-3555193

2.  CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP

     NOT APPLICABLE

3.  SEC USE ONLY


4.  CITIZENSHIP OR PLACE OF ORGANIZATION

     MASSACHUSETTS

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH

5.  SOLE VOTING POWER

     0 SHARES

6.  SHARED VOTING POWER

     2,180,394

7.  SOLE DISPOSITIVE POWER

     0

8.  SHARED DISPOSITIVE POWER

     2,180,394

9.  AGGREGATED AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

     2,180,394

10. CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES

      NOT APPLICABLE

11. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

      6.16%

12. TYPE OF REPORTING PERSON

       IA




















CUSIP NO: 00163U106   13G         Page 4 of 7 Pages

ITEM 1.
    (A)  NAME OF ISSUER
         AMAG PHARMACEUTICALS INC
    (B)  ADDRESS OF ISSUER`S PRINCIPAL EXECUTIVE OFFICES
	1100 WINTER STREET
        WALTHAM MA 02451
ITEM 2.
    (A)  NAME OF PERSON FILING
         STATE STREET CORPORATION AND ANY OTHER REPORTING PERSON
         IDENTIFIED ON THE SECOND PART OF THE COVER PAGES HERETO

    (B)  ADDRESS OF PRINCIPAL BUSINESS OFFICE OR, IN NONE,
         RESIDENCE
         STATE STREET FINANCIAL CENTER
         ONE LINCOLN STREET
         BOSTON, MA 02111
        (FOR ALL REPORTING PERSONS)

    (C)  CITIZENSHIP: SEE ITEM 4 (CITIZENSHIP OR PLACE OF
         ORGANIZATION) OF COVER PAGES

   (D)  TITLE OF CLASS OF SECURITIES
         COMMON STOCK

  (E)  CUSIP NUMBER:
          00163U106
ITEM 3.
         IF THIS STATEMENT IS FILED PURSUANT TO RULE 13D-1(B), OR 13D-2(B)
         OR (C), CHECK WHETHER THE PERSON FILING IS A:
         SEE ITEM 12(TYPE OF REPORTING PERSON) OF THE COVER PAGE
         FOR EACH REPORTING PERSON AND THE TABLE BELOW, WHICH EXPLAINS
         THE MEANING OF THE TWO LETTER SYMBOLS APPEARING IN ITEM 12 OF
         THE COVER PAGES.
         SYMBOL    CATEGORY
          BK          BANK AS DEFINED IN SECTION 3(A) (6) OF THE ACT.
          IC           INSURANCE COMPANY AS DEFINED IN SECTION 3 (A) (19)
                       OF THE ACT
          IC           INVESTMENT COMPANY REGISTERED UNDER SECTION 8 OF
                       THE INVESTMENT COMPANY ACT OF 1940.
          IA           AN INVESTMENT ADVISOR IN ACCORDANCE WITH RULE
                       13D-1(B) (1) (II) (E).
          EP           AN EMPLOYEE BENEFIT PLAN OR ENDOWMENT FUND IN
                       ACCORDANCE WITH RULE 13D-1(B) (1) (II) (F) .
          HC           A PARENT HOLDING COMPANY OR CONTROL PERSON IN
                       ACCORDANCE WITH RULE 13D-1(B)(1)(II) (G).
          SA           A SAVINGS ASSOCIATIONS AS DEFINED IN SECTION 3(B)
                       OF THE FEDERAL DEPOSIT INSURANCE ACT (12 U.S.C. 1813).
          CP           A CHURCH PLAN THAT IS EXCLUDED FROM THE DEFINITION OF
                       AN INVESTMENT COMPANY UNDER SECTION 3(C)(14) OF THE




CUSIP NO: 00163U106   13G         Page 5 of 7 Pages

ITEM 4.   OWNERSHIP
          THE INFORMATION SET FORTH IN ROWS 5 THROUGH 11 OF THE COVER PAGE
          HERETO FOR EACH OF THE REPORTING PERSONS IS INCORPORATED
          HEREIN BY REFERENCE.

ITEM 5.   OWNERSHIP OF FIVE PERCENT OR LESS OF CLASS
          NOT APPLICABLE

ITEM 6.   OWNERSHIP OF MORE THAN FIVE PERCENT ON BEHALF OF ANOTHER PERSON
          NOT APPLICABLE

ITEM 7.   IDENTIFICATION AND CLASSIFICATION OF THE SUBSIDIARY WHICH
          ACQUIRED THE SECURITY BEING REPORTED ON BY THE PARENT HOLDING
          COMPANY OR CONTROL PERSON
          SEE EXHIBIT 1 ATTACHED HERETO

ITEM 8.   IDENTIFICATION AND CLASSIFICATION OF MEMEBERS OF THE GROUP
          NOT APPLICABLE

ITEM 9.   NOTICE OF DISSOLUTION OF GROUP
          NOT APPLICABLE



























CUSIP NO: 00163U106   13G         Page 6 of 7 Pages

ITEM 10.  CERTIFICATION

          BY SIGNING BELOW I CERTIFY THAT, TO THE BEST OF MY KNOWLEDGE
AND BELIEF, THE SECURITIES REFERRED TO ABOVE WERE ACQUIRED AND ARE
HELD IN THE ORDINARY COURSE OF BUSINESS AND WERE NOT ACQUIRED AND ARE
NOT HELD FOR THE PURPOSE OR WITH THE EFFECT OF CHANGING OR INFLUENCING
THE CONTROL OF THE ISSUER OF THE SECURITIES AND WERE NOT ACQUIRED AND
ARE NOT HELD IN CONNECTION WITH OR AS A PARTICIPANT IN ANY TRANSACTION
HAVING THAT PURPOSE OR EFFECT.

SIGNATURES

	   AFTER REASONABLE INQUIRY AND TO THE BEST OF HIS KNOWLEDGE AND
BELIEF, EACH OF THE UNDERSIGNED CERTIFIES THAT THE INFORMATION SET FORTH
IN THIS STATEMENT IS TRUE, COMPLETE AND CORRECT.

FEBRUARY 14, 2018
STATE STREET CORPORATION


/S/ ELIZABETH SCHAEFER
SENIOR VICE PRESIDENT


FEBRUARY 14, 2018
SSGA FUNDS MANAGEMENT, INC.



/S/ ALYSSA A. ALBERTELLI
SENIOR VICE PRESIDENT


                                   EXHIBIT 1



   THE FOLLOWING TABLE LISTS THE IDENTITY AND ITEM 3 CLASSIFICATION
OF EACH SUBSIDIARY OF STATE STREET CORPORATION, THE PARENT HOLDING
COMPANY, THAT BENEFICIALLY OWNS THE ISSUER`S SECURITIES. PLEASE
REFER TO ITEM 3 OF THE ATTACHED SCHEDULE 13G FOR A DESCRIPTION OF
EACH OF THE TWO-LETTER SYMBOLS REPRESENTING THE ITEM 3 CLASSIFICATION
BELOW.

SUBSIDIARY							ITEM 3 CLASSIFICATION
STATE STREET BANK AND TRUST COMPANY				BK
SSGA FUNDS MANAGEMENT, INC.					 IA
STATE STREET GLOBAL ADVISORS, LTD    				 IA
STATE STREET GLOBAL ADVISORS, AUSTRALIA, LIMITED         		 IA
STATE STREET GLOBAL ADVISORS (JAPAN) CO., LTD			 IA
STATE STREET GLOBAL ADVISORS TRUST COMPANY			 IA




NOTE: ALL OF THE LEGAL ENTITIES ABOVE ARE DIRECT OR INDIRECT SUBSIDIARIES
OF STATE STREET CORPORATION.






CUSIP NO: 00163U106   13G         Page 7 of 7 Pages


                             JOINT FILING AGREEMENT



IN ACCORDANCE WITH RULE 13D-1(K)(1) UNDER THE SECURITIES EXCHANGE ACT
OF 1934, AS AMENDED (THE EXCHANGE ACT), EACH UNDERSIGNED ENTITY (EACH A
COMPANY) HEREBY AGREES TO ANY AND ALL JOINT FILINGS REQUIRED TO BE MADE
ON THE COMPANY`S BEHALF ON SCHEDULE 13G (INCLUDING AMENDMENTS THERETO)
UNDER THE EXCHANGE ACT, WITH RESPECT TO SECURITIES WHICH MAY BE DEEMED
TO BE BENEFICIALLY OWNED BY THE COMPANY UNDER THE EXCHANGE ACT, AND
THAT THIS AGREEMENT BE INCLUDED AS AN EXHIBIT TO ANY SUCH JOINT FILING.
THIS AGREEMENT MAY BE EXECUTED IN ANY NUMBER OF COUNTERPARTS ALL OF
WHICH TAKEN TOGETHER SHALL CONSTITUTE ONE AND THE SAME INSTRUMENT.

IN WITNESS WHEREOF, EACH COMPANY HEREBY EXECUTES THIS AGREEMENT
EFFECTIVE AS OF THE DATE SET FORTH BELOW.


FEBRUARY 14, 2018
STATE STREET CORPORATION




/S/ ELIZABETH SCHAEFER
SENIOR VICE PRESIDENT




FEBRUARY 14, 2018
SSGA FUNDS MANAGEMENT, INC.




/S/ ALYSSA A. ALBERTELLI
SENIOR VICE PRESIDENT






Information Classification: General


Information Classification: General