Nearly one-in-ten births in both countries was to a teenager or girl even younger in 2015, the latest year for which data is available.
That is more than three times the EU average.
The data, from Eurostat, also uncovered hundreds of girls in the 10-14 age group from France, Germany, Bulgaria and Romania become mothers each year.
Experts say a repressive sexist culture and a lack of respect for women in some countries is fueling the problem.
‘Something seriously wrong’
Nearly 1,000 births in Bulgaria and Romania in 2015 were to girls between the ages of 10 and 14.
Bulgaria had nearly 300 of the young mothers – representing nearly five percent of all the country’s teenage births.
“It is extremely high and it shows to me there is something seriously wrong in the society when you have so many girls of that age,” Irene Donadio, spokeswoman for the International Planned Parenthood Federation European Network (IPFF), told Euronews.
“You have to wonder how much consent is appreciated in the country. How much real choice about sexual and reproductive autonomy and self-determination girls have, including in relation to termination?
“In some cases we are practically talking about children, so there’s a child protection failure at play too.”
It’s not just a problem in south-eastern Europe.
Hundreds of girls the same age gave birth in Germany, France and the UK – although they made up a smaller proportion of overall teenage births.
In around half of EU countries, births to girls of this age do not even reach double figures, notably in Scandinavia.
While overall teenage births fell in every EU country over the decade to 2015, the 10-14 age group was far more stubborn.
Slovakia, France, Austria, Italy and Romania all had more mothers in 2015 than a decade earlier.
The areas of Europe worse for teenage mothers
The figures show a broad trend of higher levels of teenagers giving birth in south- and north-eastern Europe.
Bulgaria, Romania, Hungary, Slovakia, Lithuania, Latvia and Poland are all among the EU’s worst when it comes to young mothers.
The UK is the only western European country in the EU’s worst 10, with three percent of births to a girl aged 10-19.
Despite this, there has been huge progress in Britain. Teenage pregnancies have dropped from 50,396 in 2005, to 26,824 a decade later.
“The UK is a great example among the countries that have been struggling with teen pregnancy,” said Donadio.
“We celebrate the UK government’s recent decision to make sex and relationships education mandatory in all schools. This is a crucial step on the right path.”
Does Romania and Bulgaria have a ‘staggering problem’?
Around 25,000 births in Romania and Bulgaria were to teenagers or young girls or one-in-ten of new arrivals.
Like the rest of the EU, both countries have seen their numbers of young mothers come down over the last decade.
But Donadio says it is unacceptable there are such disparities depending on where you live in the bloc.
“There are two countries with a staggering problem: Bulgaria and Romania,” she added. “These countries are the ones that have a lack of engagement when it comes to gender equality and education.
“It’s not a question of individual decisions, it’s a question of the systems in place. We know in Romania and Bulgaria there are many gaps in terms of their rights and that’s why these girls have to go through this.”
Estonia ‘is a role model’
Teenage births have fallen across the EU over the last decade – but the biggest drop was in Estonia.
It had 1,116 teenagers giving birth in 2005, compared to 368 ten years later.
Although as the Feminist and Women’s Studies Association points out its numbers could have been high initially because teenage pregnancies were being encouraged because of a low birth rate.
“There is an exception and that’s Estonia,” continued Donadio. “It’s done something important over a number of years in relation to access to education, information and access to care. This has brought results.
“It’s not only a question of having a very developed system like in western Europe – Estonia have shown it can be done in that region of Europe. They are doing well and they’re a role model that could be followed.
“Estonia has made a lot of progress in comparison with their neighbouring countries. Just next to them you have Lithuania which has abstinence-only programmes in its schools and Latvia which recently introduced measures that will undermine the promotion of gender equality education in schools.”
Which countries give the most cause for concern?
Donadio said she was most worried about Hungary and Poland.
The latter’s government was forced to backtrack last year on proposals to introduce further restrictions to the country’s already restrictive abortion laws: terminations are only allowed if the life of the foetus is under threat.
“It’s not just Bulgaria and Romania, we’re seeing it all over Europe, this resistance against women’s empowerment, she said.
“In Poland for example with different legislative proposals – such as abortion and limiting access to emergency contraception.
“Poland and Hungary seem to be quite worrying in terms of their general pattern around democracy and human rights, women’s sexual reproduction rights in particular.
What should be done to reduce the number of teenage mothers?
Donadio called for the EU to play more of a role by sharing best practices and fostering school education programmes that increase respect for women’s reproductive freedoms and gender equality.
“Certainly more education [would be good],” she continued. “Because boys have to understand better and girls should know more about what their rights are.
“But information is not enough. You have to ensure women’s dignity and autonomy is respected, that means looking at the legal system, health practices and the political environment to ensure that happens.
“Because while we leave our youngsters to learn about sex to pornography I don’t think we can hope for the best results.”
Your view: What do you think? Does more need to be done to tackle teenage pregnancies in your part of Europe? Have you got a story to tell about being a young mum? Contact me: email@example.com
Listen to the interview in full