The substantive laws concerning abortion in the Union of Soviet Socialist Republics (USSR) can be grouped into four time periods and legal statuses*:

• Prior to 1920: Abortion was illegal
• 1920-1936: Abortion was legal
• 1936-1955: Abortion was illegal
• 1956-1991: Abortion was legal

* Each iteration of legislation above of course had various exceptions included, so for example while abortion was legal, certain kinds of abortion were not permitted (similar to the current discussions in the United States concerning the so-called "late-term" abortion) and conversely while abortion was illegal, when used in the case of saving the woman's life, or deemed a medical necessity, it was permitted.

That brief timeline answers your first question but unsurprisingly, abortion in the Soviet Union was a complicated issue for many reasons. In Soviet jurisprudence, jurists defined prava sub'yektivnoe (which can roughly be translated as "subjective rights") as "the possibility of a person [...] to engage in some conduct, and such possibility being provided for [...] by the state." This was part of an idiosyncrasy of Soviet law whereby "moral" rights were considered separate from "legal" rights.

In many schools of legal thought, such distinctions (i.e. legislating morality) are generally somewhat frowned upon. Notable exceptions include theocratic countries which explicitly legislate morality, and for example, modern-day Russia which has anti-blasphemy laws; these were the laws used to prosecute Pussy Riot in 2015 for performing their music/protest in Christ the Savior Church, Moscow. What exactly constitutes blasphemy though is very much open to the same kind of "moral" and "legal" rights conversations engaged in by the Soviets concerning topics like abortion which is why its legal status changed so frequently. Essentially it boils down to the idea that rights are not something inherent to the human condition and it is the responsibility of the government to adjudicate exactly which rights its citizens have at any given time.

This idea is important to your question because those time periods above coincide quite nicely with changes of power (and therefore morality and therefore law) in the Soviet Union, namely:

• Prior to Bolshevik rule in Russia, abortion was considered a fairly ghastly sin by the Orthodox Christian Church, which had a large influence on the ruling Romanov emperors and therefore a large hand in crafting law. Ecclesiastical law written in the twelfth century described abortion thus: "Women who have committed infanticide by the destruction of the child within herself or through consumption of a potion intended to do the same are to be severely punished. The church permits a husband to enact this punishment upon discovering a wife who has done this as well."
• When the Bolshevik Revolution succeeded (for these purposes let's say 1917-- although the civil war dragged on for many years afterward), the entire legal codex of the Russian Empire was discarded and rebuilt from the ground up; many things which were illegal under the Romanovs and Provisional Government became de facto legal (such as abortion and homosexual intercourse), but the Communist Party of the Russian Soviet Federative Socialist Republic (RSFSR) indeed went on to confirm the legality of abortion in 1920 becoming the first country in the world to explicitly legalize the practice. I suppose if you were looking for any kind of material encouragement from the Soviet government it would be this: abortions were provided with no questions asked by trained medical professionals in hospitals for little (or in many cases, no) cost whatsoever. The only real "encouragement" women were given vis-a-vis abortion was that if they were planning to have an abortion, it ought to be performed by a medical professional as opposed to the more generally practiced midwife-performed abortion which was dangerous to say the least because this midwife typically amounted to a woman who held dubious credentials that qualified her to perform such an operation. I have never seen any kind of "go get abortion so you can serve glorious Soviet Union in arms factory" propaganda, but I have seen Soviet propaganda which states the exact opposite ("And I wanted to have an abortion!").
• Iosef Stalin explicitly made abortion illegal in 1936 for two primary reasons: 1. Throughout his rule, despite the mass terror inflicted upon his subjects, Stalin always maintained a very particular brand of small-town morality which considered abortion to be an abomination (he had trained for several years in a Christian seminary remember) and 2. He was intensely concerned with increasing population growth in any way possible.
• Nikita Khrushchev came to power in 1953 and during his first years enacted widespread "De-Stalinization" of the Soviet Union which sought to (at least outwardly) dismantle the terror state created during Stalin's reign. Among these reforms was the liberalization of abortion laws (twenty years before Roe v. Wade keep in mind) which returned the status of abortion in the Soviet Union to one which was to be provided by trained medical professionals working on the government dime.

Sources and Further Reading

• Kotkin, Stephen; Stalin: Paradoxes of Power; 2015
• Savage, Mark; The Law of Abortion in the Union of Soviet Socialist Republics and the People's Republic of China: Women's Rights in Two Socialist Countries; Stanford Law Review; 1988