From the Guidelines
The best antibiotic to use during the first trimester of pregnancy is cephalexin, as it has been shown to be safe with no adverse effects reported, according to the most recent evidence from 2025 1. When considering antibiotic use in the first trimester of pregnancy, it's crucial to prioritize medications with established safety profiles to minimize risks to the fetus.
- Cephalexin, a cephalosporin antibiotic, is highlighted as a safe option in pregnancy, including the first trimester, with no adverse effects reported 1.
- Other antibiotics like azithromycin and clindamycin are also considered safe in pregnancy, as evidenced by studies in non-HS patients 1.
- The choice of antibiotic should be guided by the type of infection, bacterial sensitivity, and individual patient factors, ensuring the most effective treatment while minimizing potential risks.
- It's essential to avoid certain antibiotics during pregnancy, such as tetracyclines, fluoroquinolones, and trimethoprim in the first trimester, due to potential risks to fetal development.
- Always consult with a healthcare provider before taking any medication during pregnancy to ensure the best possible outcome for both the mother and the fetus.
From the FDA Drug Label
Teratogenic Effects: Reproduction studies have been performed in mice and rats at doses up to 2000 mg/kg (3 and 6 times the 3 g human dose, based on body surface area). There was no evidence of harm to the fetus due to amoxicillin. There are, however, no adequate and well-controlled studies in pregnant women Because animal reproduction studies are not always predictive of human response, amoxicillin should be used during pregnancy only if clearly needed.
The best antibiotic to use during the 1st trimester of pregnancy is amoxicillin, but it should be used only if clearly needed 2.
From the Research
Antibiotic Use During the First Trimester of Pregnancy
The use of antibiotics during the first trimester of pregnancy is a critical consideration due to the potential risks to the fetus. According to 3, beta-lactam antibiotics are generally safe and effective for use during this period.
Safe Antibiotic Options
Some of the safe antibiotic options for use during the first trimester include:
- Beta-lactam antibiotics, such as penicillins and first-generation cephalosporins 3, 4
- Macrolide antibiotics, such as erythromycin 3, 5
- Nitrofurantoin, which can be used as a second-line drug in the first trimester 5
Antibiotics to Avoid
Certain antibiotics should be avoided during the first trimester due to potential toxicity to the fetus, including:
- Fluoroquinolones 6
- Tetracyclines, except in cases where use is necessary before the 12th week of gestation 3
- Sulfonamides and chloramphenicol, which should be avoided during the prepartal period 3
- Glycopeptide and aminoglycoside antibiotics, which should be reserved for life-threatening maternal infections refractory to other antibiotics 3
Considerations for Antibiotic Use
When considering antibiotic use during the first trimester, it is essential to weigh the benefits and risks, taking into account the specific infection being treated and the potential impact on the fetus. Physiologic changes in pregnancy, such as increased glomerular filtration rate and total body volume, may lead to pharmacokinetic alterations in antibiotics, requiring dose adjustment or careful monitoring 6.