So I went through this section and there are threads about how to conceive and for women that are pregnant or going through high risk pregnancies.
But no thread for the precautions that should be taken during pregnancy.
Why?
Anyway, this thread contains information on the substances, drugs and infections that can lead to development of birth defects in unborn fetuses.
I didn't cover every single one of them, this is just a brief write-up, and a lot of the information here comes from results of findings, studies and researches done by researchers.


Birth defects occur before, after and at the time of birth of a baby. Most birth defects occur in the first 3 months of pregnancy, when the organs of the baby are forming. This is a very important stage of development. However, some birth defects occur later in pregnancy. During the last six months of pregnancy, the tissues and organs continue to grow and develop. Some birth defects (such as cleft lip or clubfoot) are easy to see, but others like heart defects or hearing loss are found using special tests, such as x-rays, CT scans, or hearing tests.

Most birth defects are thought to be caused by a complex mix of factors. These factors include our genes, our behaviors, and things in the environment. For some birth defects, the causes are known and for others the causes are unknown.
Some women have a higher chance of having a child with a birth defect:

1. Women With Certain Infections
Brain development in the fetus begins about three weeks after fertilization. This is the time when it is highly susceptible to diseases. Infections like rubella, toxoplasmosis, cytomegalovirus, and syphillis, if contracted in the first trimester, may lead to intellectual disability in a baby. Genital herpes infection increases risk of cerebral palsy, brain damnage,vision and hearing impairment.

2. Women Who Been Exposed to Environmental Toxins:
Environmental toxins can lead to mild or moderate to severe mental disabilities in the child. These include ingestion of heavy metals like cadmium. Exposure to radiations like x-rays, chemicals,etc can interfere with brain devt of the fetus.

3.Women Who Suffer From Folic Acid Deficiency:
If a mother suffers from folic acid deficiency, the infant might suffer from defects of the brain and spine also known as neural tube defects. Neural tube defects usually develop during the first month of pregnancy and to prevent it, you should not suffer from folic acid deficiency during the time of conception. it is advisable for women of childbearing age to consume about 400micrograms of folic acid a day.

4. Women with certain medical conditions, such as uncontrolled diabetes or obesity before and during pregnancy.
The risks involved in this situation include:
(i). Heart and Nervous System Defects: Women with poorly controlled diabetes in the early weeks of pregnancy are 3 to 4 times more likely than nondiabetic women to have a baby with serious birth defects like heart defects or neural tube defects (NTDs), birth defects of the brain or spinal cord.
(ii). Miscarriage: High blood sugar levels around the time of conception may increase the risk of miscarriage.
(iii). Macrosomia: Women with poorly controlled diabetes are at increased risk of having a very large baby (10 pounds or more). Macrosomia is the medical term for this. These babies grow so large because some of the extra sugar in the mother's blood crosses the placenta and goes to the fetus. The fetus then produces extra insulin, which helps it process the sugar and store it as fat. The fat tends to accumulate around the shoulders and trunk, sometimes making these babies difficult to deliver vaginally and putting them at risk for injuries during delivery.
(iv). Stillbirth: Though stillbirth is rare, the risk is increased with poorly controlled diabetes.
(v). Newborn complications: These include breathing problems, low blood sugar levels and jaundice (yellowing of the skin). These complications can be treated, but it's better to prevent them by controlling blood sugar levels during pregnancy.
(vi)A new study shows that newborns of severely or morbidly obese mothers are fatter. Furthermore, the risk of fetal macrosomy is 2.3 times greater in this group in comparison to women with a normal weight.

5. Women who take certain drugs or drink alcohol during pregnancy.
Consumption of alcohol during pregnancy can lead to the development of a condition known a s fetal alcohol syndrome in the unborn baby. Some drugs also have negative effects on infants an these drugs and their effects are listed below.

(i).Benzodiazepines: Tranquilizers, sleeping pills, and anti-anxiety drugs can cause problems in newborns including breathing, temperature regulation, and muscle problems.
(ii).Accutane and related dermatological drugs: Accutane poses an extremely serious risk of birth defects or death. This drug should never be used by a pregnant woman under any circumstances. Soriatane is also a dermatological drug that is extremely harmful to a developing fetus, and should not be used by any woman for three years prior to conception.
(iii).Aspirin can cause birth complications and birth defects when used late in pregnancy.
(iv).Antidepressants: (a)Paxil: It has been warned that Paxil can double the risk of heart birth defects when used during the first trimester of pregnancy. (b)Prozac: In 2006, a major study found that Prozac also doubled the risk of delivering a child with the birth defect called Primary Pulmonary Hypertension.
(v).Anti-cancer drugs: Teratogens such as Bulsulfan, Chlorambucil, Cyclophosphamide, Mercaptopurine, and Methotrexate can cause serious birth defects including cleft palate, underdevelopment, other physical defects and mental retardation.
(vi).Anti:seizure medications: Anticonvulsants such as Toprimate and Valproate can cause serious birth defects including spina bifida, organ deformities, cleft palate, and facial defects.
(vii).Topamax: It has been warned that theres is an increased risk of oral clefts in infants exposed to Topamax or generic rivals, during the first trimester of pregnancy. Topamax is an anti-seizure medication also approved for migraine Topamax, is a drug known to be dangerous when taken in the first trimester.
(viii).Depakote: In 2004, women were warned not to take Depakote during pregnancy because of an increased risk of birth defects and other problems. Death of the fetus, birth defects, and developmental delays, such as walking and speech delays, occurred in 28% of children whose mothers took Depakote.
(ix)Clomid: studies in animals or pregnant women have demonstrated positive evidence of fetal abnormalities. This drug should not be used in women who are or may become pregnant.
(x).Diazepam taken during the 2nd trimester can also increase risk of cleft lip or palate.

Extra Notes
[size=15pt]*[/size] If you were taking prescription medications before you became pregnant, please consult your health care provider about the safety of continuing these medications during pregnancy.

[size=15pt]*[/size]You can prevent infections and help keep your unborn baby safe. Here’s some tips on how to do that:
1. Wash your hands often with soap and water, especially when…
Using the bathroom
Touching raw meat, raw eggs, or unwashed vegetables
Preparing food and eating
Gardening or touching dirt or soil
Handling pets
Being around people who are sick
Getting saliva (spit) on your hands
Caring for and playing with children
Changing diapers
If soap and running water are not available, you can use alcohol-based hand gel.

2. Try not to share forks, cups, and food with young children.
3. Wash your hands often when around children. Their saliva and urine might contain a virus. It is likely harmless to them, but it can be dangerous for you and your unborn baby.
4. Cook your meat until it’s well done in order to kill all micro-organisms.Undercooked meats and processed meats might contain harmful bacteria.
5. Avoid unpasteurized (raw) milk and foods made from it.
6. Get tested for sexually transmitted diseases (STDs), such as HIV and hepatitis B, and protect yourself from them.
7. Avoid people that have an infection: Stay away from people who you know have infections, such as chickenpox or rubella, if you have not yet had it yourself or did not have the vaccine before pregnancy.

[size=15pt]*[/size] The first trimester of pregnancy is the period between the 1st to the 12th weeks, the second trimester is between the 13th to the 27th week and the third trimester is between the 28th to the 42nd week.