There is a general belief that any exposure during pregnancy poses a potential risk to the developing baby. In reality most women use some form of prescribed or non-prescribed medication during pregnancy and the vast majority of drugs are actually considered to be safe. 

Some definitions and basic principles

Teratogens are environmental agents introduced during pregnancy that interfere with development so that they induce or increase the incidence of a congenital (structural) malformation or birth defect. Although most recognised teratogens are drugs (prescribed or over-the-counter), they can be infections (e.g. rubella), chemicals (e.g. methyl mercury) or radiation.

Behavioural teratogens interfere with maturation and development of the embryo or fetus, resulting in functional problems such as deafness, visual loss or neuro-developmental delay. 

Some agents may cause both structural and functional problems.

Other factors that will determine whether or not a drug is a teratogen include 

  • dose of the drug
  • access to the embryo/fetus
  • genetic and other individual factors

For these reasons if you call MotherSafe you will be asked detailed questions about the exposure.

The most important issue to clarify is the timing of the exposure in the pregnancy.

Women who have a regular 28 day menstrual cycle will usually ovulate and therefore conceive around day 14 (mid-cycle).

LMP (last menstrual period) Generally a pregnancy is dated from the first day of the last menstrual period (LMP). This means that the first 2 weeks of a “pregnancy” are in fact the 2 weeks before conception. Therefore any exposure in this time is generally not considered to be a problem as it occurs before conception.

All or none period The 2 week period from the time of conception to the time of the first missed period (days 14 to 28 of the cycle), is often called the “all-or-none” period. It is generally accepted that exposures during this time do not cause malformations as the baby is just a mass of dividing stem cells and the organs have not yet started developing. (The only exceptions to this rule are the very few drugs that take several weeks to eliminate from the body.)

The time when many women realise they are pregnant is around the time of their missed period. At this stage they are said to be "4 weeks pregnant" but in fact the embryo is only 2 weeks old.

The embryonic period, or period of organ development (from 4 – 10 weeks after LMP) is the most critical period of development, during which time exposures may possibly cause structural birth defects such as spina bifida, cleft lip and palate and cardiac defects.

Most agents that have been shown to be teratogens exert their teratogenic effects during a relatively small window of time. This means that exposures outside the critical time are not a problem.

The fetal period (from 11 weeks until term) is not a high-risk time for birth defects. However, exposures in this time may affect or disrupt normally formed organs in a number of different ways, for example by causing a reduction in amniotic fluid around the baby or by reducing blood flow to the placenta.

Because the brain and nervous system continue to grow and develop throughout pregnancy drugs like alcohol that can affect these organs should still be avoided even after the first trimester. 

EDC (expected date of confinement) is initially calculated as 40 weeks from the LMP, though this may vary slightly, and is usually confirmed by a dating ultrasound.

There are always concerns about the use of new medications in pregnancy and the possibility of teratogenic effects. Because of the problems in testing new drugs in pregnant women, information about possible adverse effects must be obtained from other sources, including animal studies as well as case reports and studies following use in human pregnancy.

Categorisation of drugs in Australia

In Australia drugs are categorised (A, B1, B2, B3, C, D, or X) based on the evaluation of the available information (both human and animal).

Although the categorisation is useful as a guide for prescribing doctors and pharmacists this type of information is often not clear-cut and many health professionals will err on the side of caution and advise women to stop taking a needed medication on the basis of the categorisation. The product information provided by the drug companies also tends to be very cautious regarding use of medicines in pregnancy and breastfeeding.

Because of these concerns pregnant women and their partners may need further counselling to weigh up the potential risks and benefits of taking (or not taking) medication in their particular situation.

The MotherSafe service was established to address these specific counselling and information needs.

Counselling and perception of risk

Women counselled at MotherSafe will be told that all couples have a background risk of about 3% of having a baby born with a major birth defect, and told their own risk relative to this baseline risk.

In general the majority of exposures do not result in a risk that is significantly greater than the baseline risk. However a couple’s perceived risk of having a baby with a problem may be far greater than their actual risk. These couples will be offered a clinic appointment so that they can discuss their concerns and have their drug treatment optimised.