At Northampton Crown Court, Ratnam Yogan, aged 28, denied causing death by dangerous driving.
He and co-defendants Ramasamy Nallaikkanthan, aged 28, and 25-year-old Pushparajah Sinnappayal denied conspiring to pervert the course of justice.
Twenty-six-year-old English teacher Miss Peaty was 30 weeks pregnant at the time of the accident, on October 20 last year.
Her Vauxhall Corsa was involved in a collision in which the three defendants are alleged to have been
involved, on the A5 near Potterspury.
She was taken to Milton Keynes Hospital by air ambulance where her baby, Charlotte Louise, was delivered in an emergency caesarean operation, but Miss Peaty, from Daventry, and her daughter both died.
Yogan and Nallaikkanthan, both of Barrington Road, Manor Park, east London, and Leicester University student Sinnappayal, of Ragdale Road, Leicester, were remanded on bail until February 10 when the date for a trial will be set at Northampton Crown Court.
All three Sri Lankans came to Britain as asylum seekers because of political unrest in their own country.