There are many essential oils not recommended during pregnancy (even
if you are more than 12 weeks pregnant) because they can be toxic
over a period of time and/or affect the baby or possibly cause
miscarriage or premature labour. This is by no means a definitive
list and others may be added, but as a guide they include:
Aniseed
Basil
Birch
Bitter almond
Camphor
Cassia
Cedarwood
Cinnamon
Clary sage (used for labour only)
Clove
Comfrey
Cypress
Eucalyptus
Fennel
Hyssop
Juniper berry
Jasmine
Marjoram
Melissa
Mugwort
Mustard
Myrrh
Nutmeg
Oregano
Parsley seed
Pennyroyal
Peppermint
Rose (used for labour only)
Rosemary
Sage
Tarragon
Thyme
Wintergreen
Wormwood

Aromatherapy uses highly concentrated essential oils extracted from
flowers, herbs and trees to treat various physical and mental
conditions. Aromatherapy can be used to treat stress, anxiety,
headaches, fatigue, insomnia, depression, aches and pains,
addiction, circulation conditions, respiratory problems, nausea,
digestion and even as an aphrodisiac!

Essential oils are pure oils that are generally quite expensive.
They are sold in small, dark, coloured glass bottles and labelled
as 'pure essential oils'. (They are different from mild scented
creams or soaps.) Dosages of aromatherapy are measured in drops.
Oils are administered by adding drops to hot water in a bath,
placing some on a tissue, vaporising some for inhalation, adding
them to plain carrier oils for massage or as warm water compresses
applied to the skin. Essential oils can also be diluted in a
hydrosol spray.

Essential oils have been used by pregnant women for many years,
probably more liberally and indiscriminately than was actually safe.
In recent years, health caregivers have become more cautious about
the use of oils during pregnancy as their potent effects have been
more recognized. Their high volatility means that an aromatherapy
massage or a bath can administer the oil at higher doses because it
is absorbed through skin as well as lungs (as it evaporates into the
air). Essential oil chemicals in the bloodsteam are able to affect a
pregnant woman's body, as well as crossing the placenta and they can
be passed onto a baby through breastfeeding. It is for these reasons
some restrictions are now placed on their use.

Essential oils are very powerful and have the potential to produce
health complications for the pregnant woman and/or possibly cause
harmful side effects for her baby. Pregnant women who use essential
oils in their own work should be aware of their exposure to these
oils.

It is difficult to provide a strict guide for the use of
aromatherapy oils while you're trying to conceive, during pregnancy
and while breastfeeding. There is little research on their side
effects and much of the information available is conflicting.
Furthermore, the recommended use of certain oils changes frequently.
For example, it used to be recommended for women with morning
sickness to put peppermint oil on a tissue to inhale, it is now
thought that concentrated peppermint oil may be toxic or cause
uterine bleeding, particularly during the first 12 weeks of
pregnancy.  Be careful ladies!  

Some essential oils have the potential to induce uterine
contractions (such as clary sage and rose), therefore their use
should be limited until the woman is at least 37 weeks pregnant (or
actually in labour). Precautions or avoidance of some oils are also
recommended for certain medical conditions (such as high blood
pressure, asthma or epilepsy).

General rule is:

Never use essential oils before 12 weeks of pregnancy or if you are
trying to conceive and never use essential oils on babies under two
years of age.

For more info about herbal remedies, uses, tea recipes,
aromatherapy, safe use of herbs etc., etc.... search on secrets from
top herbal healers.