So when she went for her routine ultrasound scan at 20 weeks she was in a positive mood. Her daughter Georgina, now four, was particularly excited at the prospect of having a little brother or sister.
During the scan, however, the radiographer fell silent and took an exceptionally long time, checking every measurement. She then told Bobbie and her husband that something was seriously wrong with the baby, and that she would have to fetch the consultant.
Within 15 minutes, the Knightons, who live in Baldock, Hertfordshire, were given the news that their child - a girl - had severe spina bifida, a defect in which part of the spinal column fails to develop completely, leaving the spinal cord exposed. The baby also had a misshapen head, which suggested that she was suffering from hydrocephalus (water on the brain).
Mrs Knighton went into hospital the next day for a termination. "The labour lasted almost 24 hours and was much worse than my previous labours," she says, "because my body was not ready to deliver the baby. Mother Nature was telling me to hold on.
"The experience was horrific. Part of me died that day. We named the baby Ellen, and we both held her. We had a service for her and planted a rose tree in her memory.
"We have delayed having any more children because we were frightened of having another child out of grief."
One of the tragedies of this case is that it probably could have been prevented. Scientists discovered as long ago as 1991 - five years before Mrs Knighton's pregnancy - that if mothers increased their intake of folic acid (a vitamin occurring in liver, green leafy vegetables and certain other foods) around the time of conception their risk of having a baby affected by spina bifida could be reduced by two-thirds.
Yet today, more than seven years later, the spectre of spina bifida is still haunting many pregnancies. More than 1,000 women a year in the UK discover that they are carrying a baby affected by a neural tube defect (mainly spina bifida and hydrocephalus) and about 850 of them go through a traumatic and painful termination.
An obvious solution to the problem was put forward in the British Medical Journal in 1995 by Nicholas Wald, professor of environmental and preventive medicine at St Bartholomew's Hospital, London, who suggested that flour manufacturers should be required to fortify flour with folic acid to ensure that all women of child-bearing age consume a high enough dose to reduce the risk.
Flour is already fortified with calcium, iron, niacin and thiamine, so adding one more vitamin would not be such a big step. The then-Conservative government, however, decided to opt for a less interventionist policy. It mounted a publicity campaign to persuade women to increase their intake of folic acid.
That campaign has now come to an end, and, despite having won an international prize from the World Health Organisation, it has not yet had the desired effect. It cost more than pounds 2.3m and there is no evidence that it has significantly reduced the number of affected pregnancies.
The reason is simple: the women most at risk of having a baby with spina bifida are those who eat a diet lacking in natural vitamins and these are the same women who are the least likely to have heard of the campaign. Also, half of all pregnancies are unplanned, so most women do not start taking a supplement, such as iron or folic acid, until after a positive pregnancy test, by which time it is too late to prevent neural tube defects. Even well-educated women like Mrs Knighton often do not start taking it until two months into pregnancy.
"The current strategy of encouraging women to take supplements does not reach those women who become pregnant accidentally," said Tony Britton, spokesman for the Association for Spina Bifida and Hydrocephalus (Asbah).
"There is enough known about the safety of folic acid for the Government to require millers to put folic acid into flour just as the Food and Drug Administration in the US has done since January."
That is a view with which many specialists concur. Dr Richard Smith, editor of the BMJ, recommended the same thing in his Edwin Chadwick lecture on public health last month and Dr Patricia Troop, director of public health for the Anglia and Oxford Region, believes that a majority of regional public health directors also favour it. Professor Richard Smithells, a retired professor of paediatrics at Leeds University, who was the first person to prove a connection between maternal folic acid intake and neural tube defects, back in 1965, also wants to see folic acid routinely added to flour.
So why hasn't it happened?
In the early Nineties, the philosophy of the government was to blame. It adopted an approach of raising awareness and encouraging dietary supplements instead. But in 1997 the incoming Labour government appointed the first- ever minister of public health, and things looked likely to change.
Professor Sir John Grimley Evans, who chairs the government's subgroup on folic acid, and is a member of the Committee on the Medical Aspects of Food (Coma), says there are two important reasons why the decision to fortify flour has been delayed.
"First, there is concern about the interaction between folic acid and pernicious anaemia, a condition caused by a deficiency of vitamin B12, which can lead to neural damage. If people who are developing pernicious anaemia take too much folic acid, it can mask the anaemia, but lead to neurological damage before its true cause - vitamin B12 deficiency- is discovered."
Professor Wald believes that this problem can be overcome by teaching doctors to diagnose pernicious anaemia with greater clinical precision, while other experts believe the proposed level of folic acid enrichment would not be enough to cause problems.
The Government's subgroup on folic acid is expected to clear up doubts in this area shortly.
Unfortunately, the issue has more recently been clouded by research into other effects of folic acid. Scientists have discovered new evidence suggesting that if adults increase their intake of folic acid, they may reduce their risk of cardiovascular disease. The Government's experts on folic acid are now wondering whether this matter should be clarified before making any recommendation on fortifying food.
"There has not been a controlled trial to show that giving folic acid reduces cardiovascular disease," commented Professor Sir John Grimley Evans.
"If flour were fortified with folic acid, it would be impossible to conduct such a trial in the UK, or to discover the ideal amount of folic acid to give," he explained, "because everyone would be consuming more in their diet."
Sir John admitted that a trial to clarify the effects of folic acid on cardiovascular disease could take 10-15 years to complete. "There are obviously some people to whom neural tube defects are the only things that matter," he pointed out. "But we have to take to heart all the implications of fortifying food."
While academics puzzle over the new dilemma of folic acid and cardiovascular disease, more than 150 mothers a year will continue to give birth to babies with spina bifida and another 850 will go through the painful trauma of a late termination of a wanted baby.
Helen Brinton, MP for Peterborough, who has been campaigning on the issue, said, "It is really outrageous that the last government dragged its heels on this. How long will women have to wait?"
Antenatal Results and Choices (formerly Support Around Termination for Foetal Abnormality): telephone helpline 0171-631 0285Reuse content