¶ 1. Defendant Gorden Starkey appeals from a family court order requiring him to pay college tuition for Kristen O’Connell-Starkey. In so requiring, the family court concluded that the latest amended divorce order was not ambiguous and found that, although it left the issue of college tuition open for contest, defendant had never contested it. Therefore, defendant remained obligated under a previous agreement to pay 60% of the child’s college tuition. We affirm.
¶2. The following facts are unconstested. On June 5, 1985, plaintiff gave birth to a child while she and defendant were living together. On November 28, 1986, the parties married. In October 1993, when the child was eight years old, plaintiff filed for divorce. In November 1993, the parties stipulated to a temporary child support order in which they agreed to share legal and physical parental rights and responsibilities. That order established that the child was a child of the marriage. On June 30, 1994, the family court entered a final order based on the parties’ stipulation. The final order maintained the provisions of the temporary order.
¶ 3. In April 1996, plaintiff moved to Virginia and, pursuant to provisions made in the final order, defendant took sole physical custody of the child. Plaintiff retained shared legal custody and visitation with the child during the summer. In December 1997, defendant moved to modify the child support order, seeking child *12support from plaintiff because of her move. The parties began negotiating an agreement regarding child support. From a DNA test dated March 9, 1998, defendant learned that there was a 99% probability that he was not the child’s father. Nevertheless, on March 25, 1998, the family court approved a stipulated settlement of defendant’s child support motion. The settlement agreement contained no mention of the DNA test. The amended order awarded child support to defendant, reduced plaintiff’s share of the child’s medical expenses, required both parties to maintain life insurance policies for the child’s benefit, and required defendant to pay 60% and plaintiff to pay 40% of the child’s college tuition, room, board, and fees.
¶ 4. Approximately three months later, in June 1998, plaintiff moved to modify the child support order. She requested sole legal and physical custody so that the child could move to Virginia and attend school there. Defendant opposed the motion, arguing that there had been no real, substantial and unanticipated change in circumstances. He further argued that the change was not in the child’s best interests. On September 2, 1998, the family court entered a temporary order, again based upon a stipulation of the parties, in which the parties continued their shared legal rights and responsibilities, but agreed that plaintiff would have sole physical rights and responsibilities for the 1998-1999 school year. The order relieved plaintiff of her child support obligation, and provided that defendant would not pay child support for the next year in exchange for his not seeking an alleged child support arrearage. It affirmed all other provisions of the previous orders. During the rest of 1998 and most of 1999, the parties attempted to negotiate a final agreement to resolve plaintiff’s June 1998 motion to modify parental rights and responsibilities. On December 1, 1999, the family court approved a negotiated agreement that purported to do so and issued the requested amended order. The agreement contains three substantive paragraphs that read as follows:
1. The final order dated June 30, 1994 in the above entitled matter shall be further amended only by striking in its entirety paragraph 2 of said order. Only Paragraphs 1, 6 and 7 of the stipulated modification order dated March 24, 1998 shall be vacated and of no further force and effect. The intent of the parties here is to amend the final order dated June 30, 1994 as amended to *13reflect the fact that the Defendant Gorden Seth Starkey is not the biological father of the parties’ minor child based upon a DNA parentage test report dated March 9, 1998 and as a result as of the date of this stipulation, has no legal or physical rights and responsibilities with respect to the parties’[] minor child.
2. The stipulated temporary order dated September 1, 1998 shall be vacated in its entirety and of no further force and effect except that Plaintiff shall be entitled to claim the parties’ minor child as a dependent for income tax purposes on her 1999 tax return and thereafter.
3. The issues of future child support, health insurance, college expenses, and maintenance of life insurance, are not waived by either party.
¶ 5. In December 2003, the child finished her first semester at the University of North Carolina Wilmington. In February 2004, plaintiff filed a motion to enforce the March 25, 1998 order. She made no mention of the December 1999 agreement. Plaintiff also sought a contempt citation against defendant for violating a court order by failing to pay 60% of the child’s college expenses. Defendant opposed the motion to enforce, arguing that the 1999 agreement stripped him of all legal and physical rights and responsibilities, and therefore, he had no responsibility to pay. On June 9, 2004, Judge Keller ruled that the 1999 order was ambiguous, and he set the matter for a hearing to determine its meaning. The hearing took place over two days, November 17, 2004 and January 10, 2005, before Judge Joseph. On March 22, 2005, Judge Joseph ruled that the 1999 order was unambiguous. The family court found that the 1999 order struck only certain paragraphs of the previous agreement and left others in force. The court found that the college tuition paragraph was not stricken. Further, the family court interpreted the last paragraph of the 1999 order as specifically leaving open for dispute the issues of future child support, health insurance, college expenses, maintenance, and life insurance. Since neither party had moved to modify the March 1998 order regarding college expenses, the family court found that “the college expenses provision of the 1998 Amendment is still in effect and the defendant must pay 60% of [the child’s] college expenses.” The court also granted plaintiff interest and attorney’s fees.
*14¶ 6. On appeal, defendant raises three issues. Defendant contends that the trial court erred in concluding that the 1999 order was unambiguous. Defendant also suggests that the trial court misconstrued the 1999 order, and finally, defendant argues that the trial court erred in enforcing the March 1998 order against him.
¶ 7. We first take up defendant’s claim that the trial court erred in concluding that the 1999 order was unambiguous. The trial court reversed its earlier decision on this issue, stating that the “law of the case” doctrine is a discretionary rule of practice, not a rule of law, and a trial court has discretion to depart from the doctrine. State v. Malshuk, 2004 VT 54, ¶ 7, 177 Vt. 475, 857 A.2d 282 (mem.). The family court then determined that the last section of the 1999 order was not ambiguous when read as part of the whole agreement between the parties. The court read the 1999 order as a whole, considering it in light of the other provisions. The court particularly emphasized the passage that struck only paragraphs 1, 6, and 7 of the 1998 order:
The college expenses provision is found in paragraph 3 of the 1998 Amendment; it was not vacated or stricken by the 1999 Amendment. There are only three numbered paragraphs in the 1999 Amendment. Because the Amendment contains provisions which either vacate or strike provisions from previous orders, the defendant’s argument that the third paragraph of the 1999 Amendment was intended to vacate the third paragraph of the 1998 Amendment is just not plausible. The 1999 Amendment merely states that issues concerning college expenses and other support items can be contested in the future. When considered in the context of the entire document, there is no ambiguity in the contested paragraph of the 1999 Amendment. That portion of the 1998 Amendment in which the defendant agreed to pay 60% of [the child’s] college expenses is still in effect.
¶ 8. ‘We review de novo the trial court’s determination as to whether the agreement is ambiguous, as well as its construction of the terms of the agreement.” Downtown Barre Dev. v. C & S Wholesale Grocers, Inc., 2004 VT 47, ¶ 8, 177 Vt. 70, 857 A.2d 263. Ambiguity exists where the disputed language will allow more than one reasonable interpretation. Isbrandtsen v. N. Branch *15Corp., 150 Vt. 575, 577, 556 A.2d 81, 83 (1988). In such cases, this Court may consider the circumstances surrounding the making of the agreement to ascertain the parties’ true intent. Id. at 578, 556 A.2d at 84. While we may examine the surrounding circumstances to determine whether a provision of the agreement is ambiguous, this Court may not use those circumstances “to vary the terms of an unambiguous writing.” Kipp v. Chips Estate, 169 Vt. 102, 107, 732 A.2d 127, 131 (1999). ‘Where the terms of [an agreement] are plain and unambiguous, they will be given effect and enforced in accordance with their language.” KPC Corp. v. Book Press, Inc., 161 Vt. 145, 150, 636 A.2d 325, 328 (1993).
¶ 9. In this case, the trial court correctly concluded that the final paragraph in the 1999 amended order was not ambiguous. The issues identified in that paragraph — future child support, health insurance, college expenses, and maintenance of life insurance — are discussed in paragraphs 2, 3, 4, and 5 of the 1998 amended order. These paragraphs were not disturbed by the 1999 order. The final paragraph specifically reserves to each party the right to continue to litigate those issues, and the language “not waived by either party” puts each party on notice that he or she might still be called to account for those provisions. The disputed language will not allow more than one reasonable interpretation. Isbrandtsen, 150 Vt. at 577, 556 A.2d at 83.
¶ 10. Defendant next argues that even if the final paragraph of the 1999 order was not ambiguous, the trial court misconstrued it. We review the trial court’s construction of the parties’ agreement de novo. Four Oaks Conservation Trust v. Bianco, 2006 VT 6, ¶ 5, 179 Vt. 597, 892 A.2d 258 (mem.). In interpreting a contract we seek to effectuate the intent of the parties, Sullivan v. Lochearn, Inc., 143 Vt. 150, 152, 464 A.2d 745, 746 (1983), and “when the language of the contract is clear on its face, we will assume that the intent of the parties is embedded in its terms.” In re Verderber, 173 Vt. 612, 615, 795 A.2d 1157, 1161 (2002) (mem.).
¶ 11. Here, defendant contends that the court erred by interpreting the March 1998 and the 1999 agreement and resulting orders together as one contract. He argues that nothing in the 1999 order indicates his willingness to pay for college expenses, and therefore, that the trial court erred by reading the terms of the 1998 order into the 1999 order. Defendant argues that the two *16orders were made in such different circumstances and contain such different subject matter that the two cannot be read together to create a consistent whole.
¶ 12. We disagree. The 1999 order carefully left specific portions of the 1998 order in place. Specifically, the 1999 order did not disturb: paragraph 2, requiring defendant to maintain medical insurance for the minor child with each party paying a portion of uncovered medical expenses; paragraph 3, requiring the parties to share the cost of college education; paragraph 4, eliminating the requirement that the parties maintain the child on retirement programs; and paragraph 5, requiring the parties to maintain life insurance. That the 1999 order also stated an understanding that those issues could be revisited by the parties in the future is perfectly sensible. Because the 1999 order specifically vacated portions of the final divorce decree and of the 1998 agreement between the parties, we cannot accept defendant’s suggestion that the third paragraph was meant to replace or vacate the college expenses provision in the 1998 order. The parties clearly demonstrated their ability to amend those paragraphs that they wanted to amend, and to affirm those obligations that each wanted the other to uphold.
¶ 13. Defendant correctly notes that the two orders settled different claims. The 1998 order resolved defendant’s motion to modify child support pursuant to 15 V.S.A. § 663. The 1999 order resolved plaintiffs motion to modify child support and parental rights and responsibilities. Id. § 665. Despite their differences, both orders modified the final divorce decree. For these reasons, we find no error in the family court’s construction of the 1999 order.
¶ 14. The dissent attempts to extrapolate from the circumstances of this case support for its decision that the agreement of the parties is ambiguous. However, the assumptions made are incorrect and derive from the dissent’s basic, harsh philosophy. Because the parties stipulated that defendant was not the biological father and that he had no “legal or physical rights and responsibilities with respect to the parties’ minor child,” the dissent assumes defendant’s concession means defendant had no intention of remaining a supportive parent in the life of a child he raised as his own until the age of fourteen. There is nothing inconsistent about having no legal rights and responsibilities for a child and continuing to love and provide for that child.
*17¶ 15. Looking at what it views as the parties’ circumstances, the dissent concludes that plaintiffs choice not to return to the family court for an order of child support “is a clear indication that she believed she could not obtain child support in light of the 1999 agreement.” We cannot begin to know the reasons plaintiff did not seek any action by the court regarding child support. The fact is, she did return to court to enforce the agreement for sharing college expenses, in light of the 1999 agreement. The dissent’s assumption is incoherent on this point.
¶ 16. Finally, the dissent finds support for its finding of ambiguity because the 1999 order eliminated the entire paragraph of the original order establishing shared parental rights and responsibilities for the minor child, and suggests “the only provision left giving mother parental rights” is her right to claim the parties’ minor child as a dependent for income tax purposes. While the 1999 order did strike the shared parental rights and responsibility determination, it does not follow that mother no longer retains those rights. The stated purpose of the 1999 order amending the 1994 final order is to reflect the fact that defendant was “not entitled to any legal physical rights and responsibilities of the parties’ minor child.” Its purpose was not to remove plaintiffs rights.
¶ 17. Defendant also argues that he cannot be obligated to support the college education of a child to whom he is not biologically related and for whom he has relinquished all parental rights and responsibilities. Specifically, he argues that either a finding of paternity or a presumption of paternity arising under a final order of divorce is a necessary predicate to an ex-husband’s obligation to pay child support. Defendant concedes that the final divorce decree established paternity, created a compulsory legal duty to pay child support, and formed the legal and jurisdictional basis for the 1998 order. He contends, however, that the 1999 order, memorializing the fact that “defendant ... is not the biological father of the parties’ minor child” removed the necessary legal predicate for imposing future child support obligations.
¶ 18. This argument overlooks the fact that the parentage determination in a final divorce decree is not open to collateral attack in a motion to modify child support. Godin v. Godin, 168 Vt. 514, 523-24, 725 A.2d 904, 910 (1998); St. Hilaire v. DeBlois, 168 Vt. 445, 448, 721 A.2d 133, 135 (1998); Lerman v. Lerman, 148 *18Vt. 629, 629, 528 A.2d 1121, 1122 (1987) (mem.). Instead, the final divorce order establishes parentage, and unlike parental rights and responsibilities and child support, the family court does not retain jurisdiction of parentage once the nisi period has run. Jones v. Murphy, 172 Vt. 86, 89, 772 A.2d 502, 504 (2001). A party wishing to contest parentage after the nisi period must move to amend the judgment pursuant to Vermont Rule of Civil Procedure 60. Id.
¶ 19. In Jones v. Murphy, this Court faced a situation in which the former spouses attempted to undo a parentage determination by means of a stipulation. Id. at 87, 772 A.2d at 503. There, a DNA test, taken just weeks after the final divorce decree, demonstrated that the husband was not the father of the youngest child in the family. After the nisi period had run,1 the trial court entered an order on a stipulation amending the judgment to state that the husband was not the biological father of the child, and therefore had “no parental rights and responsibilities with respect to [the child].” Id. The mother then filed a parentage action against the biological father; the trial court found for the mother and the biological father appealed. We held that the amendment to the final divorce decree was a nullity because neither party moved to amend the judgment during the nisi period, and neither party had moved to amend the judgment under Rule 60. Id.
¶ 20. This case bears some resemblance to Jones v. Murphy, in that the parties apparently expected, by means of the 1999 agreement, to undo the parentage determination in the final divorce decree. “[T]he intent of the parties here is to amend the final order ... to reflect that the Defendant ... is not the biological father of the parties’ minor child.” The stipulation in this case was characterized as a motion to amend parental rights and responsibilities, and to the extent that the court modified the same, it was a valid exercise of the family court’s jurisdiction. 15 V.S.A. §§ 665, 668. However, as an attempt to alter the parentage determination in the original divorce decree, it is null and void. Jones, 172 Vt. at 89, 772 A.2d at 504.2 The family court loses *19jurisdiction to amend such determinations once the nisi period has run, absent a motion under Rule 60. Therefore, defendant’s argument — that the 1999 agreement removed the necessary-predicate (parentage) upon which to base the award of college expenses — fails.
¶ 21. This decision comports with the decisions in a majority of states that have examined situations in which one parent challenges a child support order by presenting DNA evidence that the parent is not biologically related to the child. For example, numerous states preclude ex-husbands from denying paternity in post-divorce proceedings when the divorce decree establishes the child as issue of the marriage.3 Even when the biological father has been located, an ex-husband may be estopped from denying his status as the child’s parent. W. v. W., 779 A.2d 716, 722 (Conn. 2001).
¶22. For the foregoing reasons, we hold that the family court correctly concluded that the agreement is unambiguous, and properly construed the parties’ agreement regarding college expenses. We further hold that insofar as the parties intended the 1999 agreement to alter the parentage determination in the final divorce decree, they failed. However, nothing in this opinion should be construed to foreclose defendant’s ability to move to modify the college expenses provision. It is clear that the 1999 order left “issues of future child support, health insurance, [and] college expenses” open to modification.4
Affirmed.
The nisi period in Vermont is three months. 15 V.S.A. § 554.
See also In re M.Z., 472 N.W.2d 222, 223 (N.D. 1991) (stipulation of paternity in support judgment precluded parties from relitigating the issue). But see Opland v. Kiesgan, 594 N.W.2d 505, 512 (Mich. Ct. App. 1999) (holding that an order based on a stipulation of the parties to modify the parentage provision of a divorce order *19is valid and prevents the putative father from raising judicial estoppel in a later paternity action).
See In re Marriage of Hotz, 214 Cal. Rptr. 658, 660 (Ct. App. 1985); W. v. W., 728 A.2d 1076, 1080-81 (Conn. 1999); S.R.D. v. T.L.B., 174 S.W.3d 502, 508 (Ky. Ct. App. 2005); Paternity of Cheryl, 746 N.E.2d 488, 499 (Mass. 2001); In re Marriage of Holland, 730 P.2d 410, 411 (Mont. 1986); Callison v. Naylor, 777 P.2d 913, 916 (N.M. Ct. App. 1989); Shondel J. v. Mark D., 853 N.E.2d 610, 614 (N.Y. 2006); In re Gilbraith, 512 N.E.2d 956, 961 (Ohio 1987); State ex rel. Russell v. West, 115 S.W.3d 886, 892 (Tenn. Ct. App. 2003); In re R.J.P., 179 S.W.3d 181, 186 (Tex. App. 2005); Marriage/Children of: Betty L.W. v. William E.W., 569 S.E.2d 77, 86 (W. Va. 2002) (per curiam); In re Paternity of JRW & KB, 814 P.2d 1256, 1264 (Wyo. 1991).
The dissent suggests that the majority opinion gives only mother, and not father, rights under paragraph 3 of the 1999 order. This is not correct. The case is here on plaintiffs motion to enforce. Defendant failed to seek modification, instead responding with a motion to strike the March 1998 order as a nullity. At the time *20of this appeal, he had not yet filed a motion to modify the provision governing college expenses.