When in pregnancy you are exposed changes what the harm is

Reproductive hazards are substances affecting the reproductive health of
men or women, and the ability of couples to have healthy children. They may be
chemical, physical or biological:

Alcohol · pesticides and other chemicals · smoking · legal and illegal drugs ·
metals such as lead and mercury · radiation · some viruses.

You can be exposed through the skin, by breathing them in, or by swallowing
them. It can happen anywhere, and is **most common in the workplace or at
home**.

What the harm depends on

Possible effects are **infertility, miscarriage, birth defects, and
developmental disabilities in children**. Which and how serious depends on:

• What the substance is
• How much you are exposed to
• How it enters your body
• How long, or how often, you are exposed
• How you react to it

The effects are different for men and women

In men, a reproductive hazard acts on the sperm — the number, the
shape, or the way they swim — and can damage the sperm's DNA. The sperm
may then be unable to fertilise an egg, or may cause problems in the
development of the fetus.

In women, a hazard can disrupt the menstrual cycle and cause **hormone
imbalance**, which raises the risk of osteoporosis, heart disease and certain
cancers, and can affect the ability to get pregnant.

The timing in pregnancy decides the harm

Same substance, different damage. The stage of pregnancy is a variable in its
own right, which is why advice about a chemical cannot be given without it.

Avoiding them

• Avoid alcohol and illegal drugs during pregnancy
• Quit smoking — and if you are not a smoker, do not start
• Take precautions with household chemicals and pesticides
• Good hygiene, including handwashing
• Follow safe work practices and procedures where there are hazards at
  your job

Source: MedlinePlus, U.S. National Library of Medicine.