I cannot and will not comment on the specific situation of the original poster ("OP"), but I will offer some general information on the topic of leaves in Ontario.
First of all, Canada is a nation of laws. Any discussion of this topic necessarily involves identifying, citing, and analysing those laws. See, e.g.,
Re Manitoba Language Rights, [1985] 1 SCR 721, 1985 CanLII 33 at ¶ 59;
Roncarelli v. Duplessis, [1959] SCR 121, 142 (Rand and Judson, JJ, concurring); preamble to the Constitution Act, 1982, Schedule B to the Canada Act 1982 (UK), 1982, c 11 ("Canada is founded upon principles that recognize ... the rule of law"); Bryan Camp,
A Brief Analysis of Governor Palin's Tax Returns for 2006 and 2007,
available on SSRN at *11 (October 5, 2008) ("So who died and made Mr. Olsen King? We are a nation of laws and you would think a [commentator] would analyze the actual law, kinda like I’ve done in this paper. It might be my analysis is wrong. But that is the point of making an analysis -- transparency.").
In Canada, employment law, including any legal right to leaves, is generally within the exclusive jurisdiction of the provincial legislatures as part of their plenary power over "Property and Civil Rights in the Province". Constitution Act, 1867, 30 & 31 Vict, c 3, § 92(13). That said, some workers are governed instead by federal law if they work in federally regulated industries or work for the federal government. The OP does not contain enough information to determine whether Ontario law or federal labour law would govern the OP's scenario. For the sake of discussion, I will consider only Ontario law.
Ontario law provides workers with two separate forms of leave related to childbirth under Part XIV of the Employment Standards Act, 2000, SO 2000, c 41 (the "SO" stands for "Statutes of Ontario"): "pregnancy leave" (up to 17 weeks unpaid, subject to conditions) and "parental leave" (up to 35 weeks unpaid, subject to conditions).
Id, §§ 46-49. An employee needs to meet all of the technical requirements of these leaves before she is entitled to any rights flowing from them. I will not discuss those conditions in this post. Assuming an employee is validly on leave, Ontario law provides that "[
u]pon the conclusion of an employee’s leave ... the employer shall reinstate the employee to the position the employee most recently held with the employer, if it still exists, or to a comparable position, if it does not", but no such right applies "if the employment of the employee is ended solely for reasons unrelated to the leave".
Id, § 53(1), (2). This statute provides the test governing this issue under Ontario law.
As mentioned, Ontario's labour laws do not apply to federally regulated jobs, which are governed by a wholly separate set of statutes (enacted by the Parliament of Canada, rather than the Legislature of Ontario).