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

                                  SCHEDULE 13G

                    Under the Securities Exchange Act of 1934
                                (Amendment No. )*

                                  NUVASIVE INC
                                (Name of Issuer)

                                     Common
                         (Title of Class of Securities)

                                    670704105
                                 (CUSIP Number)

                                December 31, 2005
             (Date of Event which Requires Filing of this Statement)

Check the following box if a fee is being paid with this statement ___. (A fee
is not required only if the filing person: (1) has a previous statement on file
reporting beneficial ownership of more than five percent of the class of
securities described in Item 1; and (2) has filed no amendment subsequent
thereto reporting beneficial ownership of five percent or less of such class.)
(See Rule 13d-7).

*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).


                                Page 1 of 6 pages


<PAGE>

CUSIP No. 670704105                     13G                    Page 2 of 6 Pages

--------------------------------------------------------------------------------
1     NAME OF REPORTING PERSON
      S.S. or I.R.S. IDENTIFICATION NO. OF ABOVE PERSON

      William Blair & Company, L.L.C.
      36-2214610
--------------------------------------------------------------------------------
2     CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP*

                                                             (a) _______
                                                             (b) _______
--------------------------------------------------------------------------------
3     SEC USE ONLY


--------------------------------------------------------------------------------
4     CITIZENSHIP OR PLACE OF ORGANIZATION

      222 W. Adams
      Chicago, Illinois 60606
--------------------------------------------------------------------------------
                     5     SOLE VOTING POWER

  NUMBER OF                1,575,426
   SHARES            -----------------------------------------------------------
BENEFICIALLY         6     SHARED VOTING POWER
  OWNED BY
    EACH                   -0-
 REPORTING           -----------------------------------------------------------
  PERSON             7     SOLE DISPOSITIVE POWER
   WITH
                           1,575,426
                     -----------------------------------------------------------
                     8     SHARED DISPOSITIVE POWER

                           -0-
--------------------------------------------------------------------------------
9     AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON

      1,575,426
--------------------------------------------------------------------------------
10    CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*

      |_|
--------------------------------------------------------------------------------
11    PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9

      6.45%
--------------------------------------------------------------------------------
12    TYPE OF REPORTING PERSON*

      BD, IA
--------------------------------------------------------------------------------

                      *SEE INSTRUCTION BEFORE FILLING OUT!


                                Page 2 of 6 pages


<PAGE>

                          INSTRUCTIONS FOR SCHEDULE 13G

INSTRUCTIONS FOR COVER PAGE

      (1) Names and Social Security Numbers of Reporting Persons--Furnish the
full legal name of each person for whom the report is filed--i.e., each person
required to sign the schedule itself--including each member of a group. Do not
include the name of a person required to be identified in the report but who is
not a reporting person. Reporting persons are also requested to furnish their
Social Security or I.R.S. identification numbers, although disclosure of such
numbers is voluntary, not mandatory (see "SPECIAL INSTRUCTIONS FOR COMPLYING
WITH SCHEDULE 13G," below).

      (2) If any of the shares beneficially owned by a reporting person are held
as a member of a group and such membership is expressly affirmed, please check
row 2(a). If the membership in a group is disclaimed or the reporting person
describes a relationship with other persons but does not affirm the existence of
a group, please check row 2(b) [unless a joint filing pursuant to Rule
13d-1(e)(1) in which case it may not be necessary to check row 2(b)].

      (3) The third row is for SEC internal use; please leave blank.

      (4) Citizenship or Place of Organization--Furnish citizenship if the named
reporting person is a natural person. Otherwise, furnish place of organization.

      (5)-(9), (11) Aggregate Amount Beneficially Owned By Each Reporting
Person, Etc.-- Rows (5) through (9) inclusive, and (11) are to be completed in
accordance with the provisions of Item 4 of Schedule 13G. All percentages are to
be rounded off to the nearest tenth (one place after decimal point).

(10) Check if the aggregate amount reported as beneficially owned in row (9)
does not include shares as to which beneficial ownership is disclaimed pursuant
to Rule 13d-4 [17 CFR 240.13d-4] under the Securities Exchange Act of 1934.

(12) Type of Reporting Person--Please classify each "reporting person" according
to the following breakdown (see Item 3 of Schedule 13G) and place the
appropriate symbol on the form:

                  Category                          Symbol
            Broker Dealer                             BD
            Bank                                      BK
            Insurance Company                         IC
            Investment Company                        IV
            Investment Adviser                        IA
            Employee Benefit Plan, Pension Fund,
             or Endowment Fund                        EP
            Parent Holding Company                    HC
            Corporation                               CO
            Partnership                               PN
            Individual                                IN
            Other                                     OO

Notes:

      Attach as many copies of the second part of the cover page as are needed,
one reporting person per page.

      Filing persons may, in order to avoid unnecessary duplication, answer
items on the schedules (Schedule 13D, 13G or 14D-1) by appropriate cross
references to an item or items on the cover page(s). This approach may only be
used where the cover page item or items provide all the disclosure required by
the schedule item. Moreover, such a use of a cover page item will result in the
item becoming a part of the schedule and accordingly being considered as "filed"
for purposes of Section 18 of the Securities Exchange Act or otherwise subject
to the liabilities of that section of the Act.

      Reporting persons may comply with their cover page filing requirements by
filing either completed copies of the blank forms available from the Commission,
printed or typed facsimiles, or computer printed facsimiles, provided the
documents filed have identical formats to the forms prescribed in the
Commission's regulations and meet existing Securities Exchange Act rules as to
such matters as clarity and size (Securities Exchange Act Rule 12b-12).

              SPECIAL INSTRUCTIONS FOR COMPLYING WITH SCHEDULE 13G

      Under Sections 13(d), 13(g), and 23 of the Securities Exchange Act of 1934
and the rules and regulations thereunder, the Commission is authorized to
solicit the information required to be supplied by this schedule by certain
security holders of certain issuers.

      Disclosure of the information specified in this schedule is mandatory,
except for Social Security or I.R.S. identification numbers, disclosure of which
is voluntary. The information will be used for the primary purpose of
determining and disclosing the holdings of certain beneficial owners of certain
equity securities. This statement will be made a matter of public record.
Therefore, any information given will be available for inspection by any member
of the public.


                                Page 3 of 6 pages


<PAGE>

      Because of the public nature of the information, the Commission can
utilize it for a variety of purposes, including referral to other governmental
authorities or securities self-regulatory organizations for investigatory
purposes or in connection with litigation involving the Federal securities laws
or other civil, criminal or regulatory statutes or provisions. Social Security
or I.R.S. identification numbers, if furnished, will assist the Commission in
identifying security holders and, therefore, in promptly processing statements
of beneficial ownership of securities.

      Failure to disclose the information requested by this schedule, except for
Social Security or I.R.S. identification numbers, may result in civil or
criminal action against the persons involved for violation of the Federal
securities laws and rules promulgated thereunder.

                              GENERAL INSTRUCTIONS

A. Statements containing the information required by this schedule shall be
filed not later than February 14 following the calendar year covered by the
statement or within the time specified in Rule 13d-1(b)(2), if applicable.

B. Information contained in a form which is required to be filed by rules under
section 13(f)(15 U.S.C. 78m(f)) for the same calendar year as that covered by a
statement on this schedule may be incorporated by reference in response to any
of the items of this schedule. If such information is incorporated by reference
in this schedule, copies of the relevant pages of such form shall be filed as an
exhibit to this schedule.

C. The item numbers and captions of the items shall be included but the text of
the items is to be omitted. The answers to the items shall be so prepared as to
indicate clearly the coverage of the items without referring to the text of the
items. Answer every item. If an item is inapplicable or the answer is in the
negative, so state.

ITEM 1.

      (a)   Name of Issuer

            NUVASIVE INC

      (b)   Address of Issuer's Principal Executive Offices

            4545 Towne Centre Court
            San Diego, CA 92121

ITEM 2.

      (a)   Name of Person Filing

            William Blair & Company, L.L.C.

      (b)   Address of Principal Business Office or, if none, Residence

            222 W. Adams Street
            Chicago, Illinois 60606

      (c)   Citizenship

            U.S.A

      (d)   Title of Class of Securities

            Common

      (e)   CUSIP Number

            670704105

ITEM 3. IF THIS STATEMENT IS FILED PURSUANT TO RULE 13D-1(B), OR 13D-2(B), CHECK
WHETHER THE PERSON FILING IS A:

      (a)   |X| Broker or Dealer registered under Section 15 of the Act

      (b)   |_| Bank as defined in section 3(a)(6) of the Act

      (c)   |_| Insurance Company as defined in section 3(a)(19) of the act

      (d)   |_| Investment Company registered under section 8 of the Investment
                Company Act

      (e)   |X| Investment Adviser registered under section 203 of the
                Investment Advisers Act of 1940

      (f)   |_| Employee Benefit Plan, Pension Fund which is subject to the
                provisions of the Employee Retirement Income Security Act of
                1974 or Endowment Fund; see section 240.13d-1(b)(1)(ii)(F)

      (g)   |_| Parent Holding Company, in accordance with section
                240.13d-1(b)(ii)(G)(Note: See Item 7)

      (h)   |_| Group, in accordance with section 240.13d-1(b)(1)(ii)(H)

ITEM 4. OWNERSHIP

      If the percent of the class owned, as of December 31 of the year covered
by the statement, or as of the last day of any month described in Rule
13d-1(b)(2), if applicable, exceeds five percent, provide the following
information as of that date and identify those shares which there is a right to
acquire.

      (a)   Amount Beneficially Owned

            1,575,426

      (b)   Percent of Class

            6.45%


                                Page 4 of 6 pages


<PAGE>

      (c)   Number of shares as to which such person has:

            (i)   sole power to vote or to direct the vote

                  1,575,426

            (ii)  shared power to vote or to direct the vote

                  -0-

            (iii) sole power to dispose or to direct the disposition of

                  1,575,426

            (iv)  shared power to dispose or to direct the disposition of

                  -0-

Instruction: For computations regarding securities which represent a right to
acquire an underlying security see Rule 13d-3(d)(1).

ITEM 5. OWNERSHIP OF FIVE PERCENT OR LESS OF A CLASS

      If this statement is being filed to report the fact that as of the date
hereof the reporting person has ceased to be the beneficial owner of more than
five percent of the class of securities, check the following |_|.

Instruction: Dissolution of a group requires a response to this item.

ITEM 6. OWNERSHIP OF MORE THAN FIVE PERCENT ON BEHALF OF ANOTHER PERSON.

      If any other person is known to have the right to receive or the power to
direct the receipt of dividends from, or the proceeds from the sale of, such
securities, a statement to that effect should be included in response to this
item and, if such interest relates to more than five percent of the class, such
person should be identified. A listing of the shareholders of an investment
company registered under the Investment Company Act of 1940 or the beneficiaries
of employee benefit plan, pension fund or endowment fund is not required.

ITEM 7. IDENTIFICATION AND CLASSIFICATION OF THE SUBSIDIARY WHICH ACQUIRED THE
        SECURITY BEING REPORTED ON BY THE PARENT HOLDING COMPANY

      If a parent holding company has filed this schedule, pursuant to Rule
13d-1(b)(ii)(G), so indicate under Item 3(g) and attach an exhibit stating the
identity and the Item 3 classification of the relevant subsidiary. If a parent
holding company has filed this schedule pursuant to Rule 13d-1(c), attach an
exhibit stating the identification of the relevant subsidiary.

ITEM 8. IDENTIFICATION AND CLASSIFICATION OF MEMBERS OF THE GROUP

      If a group has filed this schedule pursuant to Rule 13d-1(b)(ii)(H), so
indicate under Item 3(h) and attach an exhibit stating the identity and Item 3
classification of each member of the group. If a group has filed this schedule
pursuant to Rule 13d-1(c), attach an exhibit stating the identity of each member
of the group.

ITEM 9. NOTICE OF DISSOLUTION OF GROUP

      Notice of dissolution of a group may be furnished as an exhibit stating
the date of the dissolution and that all further filings with respect to
transactions in the security reported on will be filed, if required, by members
of the group, in their individual capacity. See Item 5.

ITEM 10. CERTIFICATION

      The following certification shall be included if the statement is filed
pursuant to Rule 13d-1(b):

      By signing below I certify that, to the best of my knowledge and belief,
the securities referred to above were acquired in the ordinary course of
business and were not acquired for the purpose of and do not have the effect of
changing or influencing the control of the issuer of such securities and were
not acquired in connection with or as a participant in any transaction having
such purposes or effect.


                                Page 5 of 6 pages


<PAGE>

                                    SIGNATURE

      After reasonable inquiry and to the best of my knowledge and belief, I
certify that the information set forth in this statement is true, complete and
correct.

                                                              2-13-06
                                                  ------------------------------
                                                               Date

                                                        /S/ Michelle Seitz
                                                  ------------------------------
                                                             Signature

                                                      Principal & Manager of
                                                  Investment Management Services
                                                  ------------------------------
                                                            Name/Title

      The original statement shall be signed by each person on whose behalf the
statement is filed or his authorized representative. If the statement is signed
on behalf of a person by his authorized representative other than an executive
officer or general partner of the filing person, evidence of the
representative's authority to sign on behalf of such person shall be filed with
the statement, provided, however, that a power of attorney for this purpose
which is already on file with the Commission may be incorporated by reference.
The name and any title of each person who signs the statement shall be typed or
printed beneath his signature.

Note: Six copies of this statement, including all exhibits, should be filed with
the Commission.

      ATTENTION: INTENTIONAL MISSTATEMENTS OR OMISSIONS OF FACT CONSTITUTE
FEDERAL CRIMINAL VIOLATIONS (SEE 18 U.S.C. 1001)


                                Page 6 of 6 pages