While obviously I sympathise with your son feeling betrayed and angry about his girlfriend's feckless behaviour, I'm surprised his reaction has been quite as strong as it is. I'm wondering, to be honest, whether he hasn't been looking for an "out" to his marriage for a long time, and at last he's got an excuse to lay the blame at his girlfriend's door, without sharing any responsibility. And responsibility is something your son seems hell-bent on avoiding. How does he think his poor two-year-old is feeling back home? The child is an innocent party in all of this. Why has his daddy suddenly disappeared? Is it his fault?

And how does he think the baby will feel when it's born to find it has no father around? No doubt his or her mother will be fairly distressed, anyway. Does your son not realise that he shares responsibility for this event? Would he have reacted this badly had it been a genuine mistake – no contraception is totally reliable. If he felt so strongly, why didn't he have a vasectomy? Or use a condom, at least?

And now the mistake's been made – his partner almost certainly thought things would settle down after a bit of an upset – can't he realise that this new baby is still his, that he owes it something? It is his child that's been conceived, your grandchild, and his son's brother or sister. Is your son so intent on using their money to afford foreign holidays and more freedom that he prefers this to having another loving little person in his life?

I'm almost starting to wonder whether your son isn't, actually, on the verge of a nervous breakdown. Perhaps he has already been working all God's hours and wondering how he could even manage to bring up a family of one, and the idea of another mouth to feed has toppled him into near-insanity.

After all, it's not as if his partner had a passionate affair with his best friend, or sold the family jewellery to fund a drug habit, or has been abusive or turned into a raging alcoholic. All she's done is do something natural – get pregnant with her partner's child, partly, perhaps, because of the intensity of the mothering instinct and partly, surely, because she loves your son.

Your role, Francesca, is extremely important here. There is nothing more healing than a sympathetic third party to ease the way between warring factions. Why don't you talk to your son's partner to find out exactly what her side of the story is? Were any marital problems before this violent split? Find out how far she is prepared to go to get him back. Is there perhaps any financial help you could offer the family to ease the stress of it all for your son?

Hint to your son that he can't stay with you for ever, and emphasise how much his family is missing him. Explain that he must share the blame, if only because he had sex with his partner. And tell him, firmly but kindly, that he must go back. On him depends the happiness of four people – his two children, his wife and, ultimately, himself.

Readers say

She was acting on instinct

What an awful situation. I knew a woman who tried a similar trick to become pregnant against the wishes of her partner. I thought less of her for it. The thing is, matters to do with having children are beyond reason. It's a biological, hormonal, evolutionary and entirely irrational instinct.

Yes, Francesca's daughter-in-law did the wrong thing. But it seems she was responding to an urge so great, she had no choice. And what was that urge for? Not money, or passion, attention, or anything selfish. She wanted to create a life, her husband's child. I understand how furious Francesca's son is, but his wife was following the basic instinct on which the human race depends. I think he'll gain so much more from supporting her than by punishing her, his children and himself by leaving his family.

Tania Payne, London, W5

Tell him to go back

Your son has behaved very badly. So what that his partner is pregnant? It could have happened by accident anyway. If the partnership was solid it would have withstood this. Tell him to go back to his partner and get on with it.

Patricia McClymont, Edinburgh

He must swallow his pride

Francesca's son is not the first person to be deceived in this way. Isn't he being a bit self-indulgent? His responsibility is to care for his partner and children for as long as they need him. He must swallow his pride, go back to them and welcome the baby. He will soon love him or her to bits. What he must not do is cause suffering to his partner deprive his children of their father just because he is "furious".

Virginia Wiltshire, Alton, Hampshire

What a spoilt brat

Francesca's son sounds like a spoilt brat. He left a little boy he claims he loves just to punish his wife who dared to get pregnant against his wishes. What a selfish man. He should go back to his family. Time to grow up and be a responsible parent.

Galina Varese, Witney, Oxfordshire

Throw your son out

I can't read the words "lost everything" without steam coming out of my ears! He hasn't lost everything, he's walked out on everything. He's thrown a tantrum because life has not delivered what he specified. One could be critical of his partner for deceiving him, but maybe she was desperate for a second child and figured he would love it. What he's done is horrific – abandoned his pregnant partner and the child he "adores", and seemingly forced the sale of the house. How does he think his partner will cope with a house move when she is in the last stages of pregnancy or has just given birth? Does she even have the resources to buy another property? Doesn't he care what happens to her and his children?

You should throw your son out of your house and tell him to go back to his home and his responsibilities. And after that, you might consider whether there is anything in his upbringing that has made him into the person he sounds to be.

Lorraine Harding, Steeton, West Yorkshire