Five-time Grand Slam winner Sharapova was suspended last March after testing positive for meldonium.
“She’s a cheater and I don’t think a cheater in any sport should be allowed to play that sport again,” said Canadian Bouchard. “It’s so unfair to all the other players who do it in the right way and are true.”
Sharapova, 30, has received wildcards for the tournaments in Madrid and Rome and will find out if she has been given one for the French Open on 16 May – 12 days before the competition.
“It wasn’t very easy,” she said. “You hear all these stories about people when they’re pregnant – they get sick, they get really tired, really stressed out.
“I had to really take all that energy and put it in a paper bag, so to say, and throw it away.
“Pregnant or not, no-one knew and I was supposed to win that tournament. Every time I play, I’m expected to win. If I don’t win, it’s actually much bigger news.”
Williams, who is taking maternity leave for the rest of the 2017 season, said there was no change to her plan to return to the tour as a mother next year.
“I definitely plan on coming back. I’m not done yet,” said Williams, who credited 36-year-old sister Venus for inspiration.
“If she’s still playing, I know I can play. This [motherhood] is just a new part of my life. My baby’s going to be in the stands and hopefully cheering for me.”
On Tuesday, Williams called Ilie Nastase’s comments about her unborn child “racist”.
Nastase, a former world number one and two-time Grand Slam winner, was heard speculating whether Williams’ child would be “chocolate with milk?” at a news conference before Romania’s Fed Cup tie with Great Britain last week.
Maria Sharapova’s wildcard entry into the Stuttgart Open has been defended by WTA chief Steve Simon who said it is in keeping with how former dopers are treated in other sports.
The 30-year-old Russian’s 15-month ban for using meldonium ends on Wednesday when she plays in the first round.
“You have to look at how other leagues and tours have handled players who have come back,” Simon told BBC Sport.
“They come right back to the team and start playing.”
Italy’s Roberta Vinci – Sharapova’s opponent in her comeback match on Wednesday evening – disagrees with the decision to invite the five-time Grand Slam champion into the main draw.
World number eight Agnieszka Radwanska, who Sharapova could meet in the second round, and former world number one Caroline Wozniacki have also questioned Sharapova’s presence at the tournament.
In addition to Stuttgart, Sharapova has been granted wildcards by the organisers of the events in Madrid and Rome. She does not have a world ranking after her points expired during her suspension and would need to reach the final in Stuttgart to be eligible for French Open qualifying.
The Daily Telegraph report that Sharapova is likely to be given a wildcard into qualifying at Roland Garros rather than the tournament’s main draw.
“She is starting at ground zero,” Simon added.
“It is going to affect her seedings in big tournaments so she’s still going to pay a penalty for a while. If you think about it from other leagues, most of them (bans) are half of a season or a full season. She’s had a year and a half.”
Maria Sharapova’s first opponent following her 15-month doping ban has questioned the decision to give the Russian wildcards on the WTA Tour.
Sharapova plays Italy’s Roberta Vinci in the first round of the Porsche Grand Prix in Stuttgart on Wednesday.
The 30-year-old’s wildcard entry has already been called “disrespectful” by ex-world number one Caroline Wozniacki.
“I don’t agree about the wildcard here and about the wildcard in Rome and the other tournaments,” said Vinci, 34.
Sharapova was given a two-year ban last year, backdated to 26 January 2016, after testing positive for heart disease drug meldonium at the Australian Open.
Her suspension was reduced to 15 months in October, following her appeal to the Court of Arbitration for Sport.
Sharapova will also receive wildcards for upcoming tournaments in Madrid and Rome.
World number 36 Vinci added: “She made her mistakes for sure, but she paid and I think she can return to play – but without any wildcards.”
Agnieszka Radwanska of Poland, who could meet Sharapova in the second round in Stuttgart, has also been among those to question the treatment of the former world number one, saying she should not be invited to Grand Slams.
Those views were met with a scathing response by Sharapova’s agent Max Eisenbud, who labelled Radwanska, 28, and 26-year-old Wozniacki of Denmark “journeyman” rivals who wanted to prevent the Russian playing at next month’s French Open because it is their “last chance to win a Slam”.
Sharapova, twice a winner at the French Open, is unranked and will require a wildcard to compete at Roland Garros when the tournament starts next month, with France’s tennis federation yet to announce its decision.