Drugs in pregnancy
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Drug use during pregnancy can have temporary or permanent effects on the fetus. Any drug that acts during embryonic or fetal development to produce a permanent alteration of form or function is known as a teratogen. Drugs may refer to both pharmaceutical drug and recreational drugs.
Many physicians prefer not to prescribe drugs for pregnant women due to concerns over teratogenicity. The apprehension is not necessarily data driven and is a cautionary response to the lack of clinical studies in pregnant women. The indication is a trade-off between the adverse effects of the drug, the risks associated with intercurrent diseases and pregnancy complications, and the efficacy of the drug to prevent or ameliorate such risks. In some cases, the use of drugs in pregnancy carries benefits that outweigh the risks. For example, high fever is harmful for the fetus in the early months, thus the use of paracetamol (acetaminophen) is generally associated with lower risk than the fever itself. Similarly, diabetes mellitus during pregnancy may need intensive therapy with insulin to prevent complications to mother and baby.
The Food and Drug Administration (FDA) has developed a rating system to provide therapeutic guidance based on potential benefits and fetal risks. Drugs have been classified into categories A, B, C, D and X based on this system of classification. Drugs, and some multivitamins, that have demonstrated no fetal risks after controlled studies in humans are classified as Category A, while drugs like thalidomide with proven fetal risks that outweigh all benefits are classified as Category X.
- Category B : Penicillin, nitrofurantoin, clindamycin, terbinafine, some macrolides e.g. azithromycin, erythromycin
- Category B : Low molecular weight heparin
- Category C : Unfractionated heparin, beta blockers, (dihydropyridine) calcium antagonists, furosemide, digoxin, methyldopa
- Category D : ACE inhibitors, angiotensin II receptor blockers (ARBs), coumarins, thiazides, diltiazem
Central nervous system drugs
- Category B : Acetaminophen,
- Category C : Vaccines such as tetanus toxoid, polio vaccine, BCG vaccine, hepatitis A vaccine, hepatitis B vaccine and rabies vaccine.
- Category X: Alcohol is one of the most dangerous drugs for a pregnant woman, especially in the early weeks of pregnancy.
Alcohol passes easily through the placenta from the mother's bloodstream into the bloodstream of the fetus. Unlike adults, a fetus does not yet have a blood filtration system, and is therefore unprotected from the effects of alcohol.
In the first two weeks following fertilization, maternal alcohol consumption may prevent proper implantation of the fertilized egg, usually terminating the pregnancy before the mother realizes she is pregnant.
Potential problems during pregnancy:
- Decreased fetal breathing
- Delayed development of the nervous system 
- Facial feature abnormalities, especially during the third through sixth weeks following fertilization
- Beginning in the fourth week - rapid cardiac development. Exposure to alcohol may result in atrial and/or ventricular abnormalities, issues with valve formation, and a potential increase in the risk for heart disease in adulthood.
- Sixth and seventh weeks - the corpus callosum, the structure that divides the left and right hemispheres of the brain and is responsible for communication between the two hemispheres, is most vulnerable to alcohol exposure. Exposure to alcohol may lead to its incomplete development, thus resulting in issues with neural communication.
- After 8 weeks gestation, the fetus is not as vulnerable to organ-specific birth defects. The central nervous system overall, however, remains vulnerable, as well as the overall growth of the fetus.
- Infertility, miscarriage, stillbirth, low birth weight, preterm delivery 
Potential effects in children exposed to alcohol during prenatal development:
- Development of a Fetal Alcohol Spectrum Disorder
- Neurological disorders - ranging from attention deficit disorder (ADD), hyperactivity, mood disorders
- Impaired attention, learning, memory, and information processing
- Poor bonding and difficulty forming social relationships
- Higher rates of antisocial and delinquent behaviors
- Difficulty with executive functions
- Very high risk of mental health disorders (92% of people diagnosed with Fetal Alcohol Spectrum Disorders have also been diagnosed with a mental health disorder) 
Fetal Alcohol Spectrum Disorder: and umbrella term used to describe a variety of potential outcomes for prenatal exposure to alcohol. These outcomes include -
- Fetal Alcohol Syndrome (FAS)
- Partial Fetal Alcohol Syndrome
- Alcohol-Related Neurodevelopmental Disorder (ARND)
- Fetal Alcohol Effects (FAE)
- Alcohol Related Birth Defects (ARBD) 
Recreational drug use during pregnancy can cause various pregnancy complications.
Cannabis in pregnancy is the subject of various scientific studies, usually regarding whether it has effects on the child later in life.
A number of studies have shown that tobacco use is a significant factor in miscarriages among pregnant smokers, and that it contributes to a number of other threats to the health of the fetus. Smoking and pregnancy, combined, cause twice the risk of premature rupture of membranes, placental abruption and placenta previa. Also, it causes 30% higher odds of the baby being born prematurely.
- Prenatal cocaine exposure is associated with, for example, premature birth, birth defects and attention deficit disorder.
- Prenatal methamphetamine exposure can cause premature birth and congenital abnormalities. Other investigations have revealed short-term neonatal outcomes to include small deficits in infant neurobehavioral function and growth restriction when compared to control infants. Also, prenatal methamphetamine use is believed to have long-term effects in terms of brain development, which may last for many years.
- Marijuana can cause low birth weight, tremors, poor eye-sight, late start of breathing, and a hole in the heart. In the first six months of life babies who have been exposed to marijuana have a higher chance of having breathing problems such as asthma, chest infections, and wheezing. By the age of three or four the child might be fearful, reckless, inattentive, restless, irresponsible, and may have poor memory, verbal, and reasoning ability; at age ten these problems can continue along with depression, anxiety, reading and spelling problems.
By pregnancy stage
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Pregnancy and fetal development progress through various changes. The period of one week from fertilisation to implantation of the fertilized egg is called the preimplantation period. This is an 'all or none' period, .i.e. an insult can either cause death or complete recovery can occur. The period from the eighth day to the end of eighth week is the period of organogenesis during which the organs are formed in the fetus. This is the most crucial time with regards to 'structural malformations' and concern over teratogenicity of drugs. From the third month to the end of nine months is the period of fetal maturation. Intake of drugs during this period may modify the 'function' of the fetal organs rather than causing gross structural malformations in the fetus; for example, aminoglycosides can affect the functioning of the kidneys as well as the hearing mechanism.
- Nathanson, Vivienne; Nicky Jayesinghe; George Roycroft (Oct 27, 2007). "Is it all right for women to drink small amounts of alcohol in pregnancy? No". BMJ 335 (7625): 857. doi:10.1136/bmj.39356.489340.AD. PMC 2043444. PMID 17962287.
- O'Neil, Erica. "Developmental Timeline of Alcohol-Induced Birth Defects". Arizona State University, School of Life Sciences. Retrieved May 1, 2014.
- Coons, Kelly (Nov 3, 2013). "Determinants of Drinking During Pregnancy and Lifespan Outcomes for Individuals with Fetal Alcohol Spectrum Disorder". Journal on Developmental Disabilities 19 (3): 15–24. Retrieved May 1, 2014.
- Ness RB, Grisso JA, Hirschinger N, et al. (February 1999). "Cocaine and tobacco use and the risk of spontaneous abortion". N. Engl. J. Med. 340 (5): 333–9. doi:10.1056/NEJM199902043400501. PMID 9929522.
- Oncken C, Kranzler H, O'Malley P, Gendreau P, Campbell WA (May 2002). "The effect of cigarette smoking on fetal heart rate characteristics". Obstet Gynecol 99 (5 Pt 1): 751–5. doi:10.1016/S0029-7844(02)01948-8. PMID 11978283.
- "Preventing Smoking and Exposure to Secondhand Smoke Before, During, and After Pregnancy" (PDF). Centers for Disease Control and Prevention. 2007.
- "Tobacco Use and Pregnancy: Home". Centers for Disease Control and Prevention. 2009.
- "New Mother Fact Sheet: Methamphetamine Use During Pregnancy" (PDF). North Dakota Department of Health. Archived from the original on 2013-03-15. Retrieved 4 February 2014.
- Grotta, S.; LaGasse, L.; Arria, A.; Derauf, C.; Grant, P.; Smith, L.M. et al. (30 June 2009). "Patterns of Methamphetamine Use During Pregnancy: Results from the IDEAL Study". Matern Child Health J 14 (4): 519–527. doi:10.1007/s10995-009-0491-0. PMC 2895902. PMID 19565330.
- University of Washington Alcohol & Drug Abuse Institute. "Marijuana". Reproduction and Pregnancy. Retrieved 2011.