The scooter Danny Glass was riding on with Sophie Emma Rose Anderson collided with a truck in Phuket on 8 May.
Mr Glass, 29, originally from Margate, escaped serious injury.
Now he and the lorry driver Natt Awut Timsue are to be charged with causing death by reckless driving, Thai police said.
Mr Glass - who lived in Thailand with Ms Anderson - is to hear the charges formally on Thursday and a translator has been called in for the hearing.
His family is expected to attend.
Ms Anderson, 41, originally from Blackpool, was a keen video blogger, as was Mr Glass.
She had a YouTube channel called Sophie's Joy Breastfeeding Mama, on which she uploaded videos about nursing her son until the age of eight.
After her death Mr Glass paid tribute to her in a video he uploaded on YouTube.
He said he "missed her so much" and added: "Somebody wake me up from this nightmare... she is meant to be having my baby in three months."
On Tuesday he posted another video titled 'When will I come back to Youtube', in which he stated he would not be posting any more videos for "a while" and did not know when he would return online.
He said: "It's not the right time at the moment, I need to get my life in order and get through this death."
A Foreign Office spokesman said: "We are in contact with the Thai police and are providing consular support to a British man following a road traffic accident in Bangkok."

The unemployment rate was 4.9%, down from 5.5% a year earlier but little changed from last month's report, the Office for National Statistics said.
Growth in average weekly earnings including bonuses slowed slightly to an increase of 2.3% on last year.
The proportion of people in work hit a record high of 74.5%.
The ONS said the figures, which only cover one month since the result of the EU referendum, show "continuing improvement" in the jobs market.
But Samuel Tombs, chief UK economist at Pantheon Macroeconomics, warned: "When you scratch beneath the surface, today's labour market figures are not as robust as they first appear."
The rise in people in work "remains supported by surging self-employment", Mr Tombs said.
"The strong growth also reflected a shift towards part-time working; total weekly hours rose by just 0.3% between April and July," he added.
Ben Brettell, senior economist at Hargreaves Lansdown, said the labour market is "still waiting for the Brexit effect", but that it will "gradually become clear over the next few months".
In August the claimant count, including Jobseeker's Allowance, actually went up from the previous month by 2,400 to 771,000.
The claimant count is treated with some caution, though, by economists as the move to Universal Credit has made it much harder to calculate.
One interesting nugget in the employment figures concerns the number of people employed in the public sector.
It is down to 5.33 million, the lowest level since the Office for National Statistics started collecting the figures in 1999 and a drop of 13,000 since March.
The former chancellor, George Osborne, often spoke about the need to "rebalance" the economy.
To an extent, he meant away from financial services and towards manufacturing.
But, more privately, he was actually very comfortable with an economy that was "rebalancing" away from the public sector towards the private sector, where wealth, he would argue, is actually created.
As cuts and wage freezes bite, the public sector has become less attractive as a place to build a career.
Yes, employment is at record levels, but it is the private sector that is on the up.
The unemployment estimates come from the Labour Force Survey in which the ONS talks to 40,000 households every three months.
Although it is a very large survey, there is still a margin of error. The ONS says it is 95% confident that the figure of a 39,000 fall in unemployment is correct to within 78,000.
As the estimated change is smaller than the margin of error, it means the change in unemployment is not statistically significant.
The boyfriend of a pregnant woman killed in a scooter accident in Thailand is to be charged over her death, police in the country have said.
[NEXT_CONCEPT]
The number of people without jobs and looking for work fell by 39,000 to 1.63 million between May and July, official figures show.