According to the Orthodox Church, they may be exempted from the full rigor of fasting. But that is a matter between the woman, her doctor, and her spiritual adviser. One way to observe the spirit of the fast while not endangering the health of the child is to eat simple foods. If she must have meat, simple cooking, with simple or no seasoning. And just enough to supply the needs of the body, not satisfy the wants of the person.
With few exceptions, every Wednesday and Friday are days of fasting, in theory only one meal a day and that after sundown (or Vespers), and no products from vertebrates - meat, milk, cheese, eggs. Some water during the day is allowed. During any of the longer periods of fasting -
Great and Holy Lent before Pascha (the 40 days before Holy Week, the first week of which there should only be two meals taken - no food from sundown on Sunday until after taking the Presanctifed Gifts on Wednesday, when one meal is taken, and then again nothing until after the Presactified on Friday),
Holy Week - during which one meal a day is taken, but many mimic the first week of Lent during Holy Week.
the Apostles Fast (of variable length, form the 2nd Sunday after Pentecost until the Feast of Sts. Peter an Paul on 29 June),
the Dormition Fast (August 1st to 14th),
and the Nativity Fast (40 days before the Nativity)
during the week one meal a day, again after sundown/Vespers and without vertebrate products, wine, or olive oil, and two meals on weekends (Sunday commemorates the Resurrection, so it and Saturday as the day of preparation are semi-festal days so two meals are allowed). All meals should be modest in quantity, just enough to quell the hunger without being sated. Included in the concept of fasting is increased almsgiving (in theory at least the amount that would normally have been spent on food), increased prayer, and fasting from such things as TV, radio, movies, etc.
Funny how we are all asked to be considerate of the mohammedeans during Ramadan, when they don't eat during the daylight hours, but have no restrictions on type or amount during the hours of darkness, but no mention is given to Christian days of fasting.