Merck Manual

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Timing and the Effects of Drugs During Pregnancy

Timing and the Effects of Drugs During Pregnancy

Time Frame

Possible Drug* Effects

Status of the Fetus

Within 20 days after fertilization

An all-or-nothing effect (death of the fetus or no effect at all)

The fetus is highly resistant to birth defects.

3‒8 weeks after fertilization

Possibly no effect

A permanent but subtle defect that is noticed only later in life

The fetus's organs are developing, making the fetus particularly vulnerable to birth defects.

2nd and 3rd trimesters

Changes in the growth and function of normally formed organs and tissue

Unlikely to cause obvious birth defects

Unknown long-term effects

Organ development is complete.

* Only certain drugs are likely to have harmful effects.