GUIDE
In week 1 of pregnancy by medical dating, you are having your period. Conception has not occurred yet — this week is counted retroactively from the first day of your last menstrual period (LMP).
It sounds confusing, but there is a good reason. Doctors date pregnancy from your LMP because most people know when their period started, while the exact day of ovulation and conception is harder to pinpoint. This means the first two weeks of pregnancy are pre-conception.
Be prepared from day one
Medical pregnancy dating begins on the first day of your last menstrual period (LMP), not on the day of conception. This means that during "week 1 of pregnancy," you are actually menstruating. It may seem counterintuitive, but this convention exists because the LMP is a known, concrete date — whereas the exact moment of conception is usually unknown.
The practical implication is that when your doctor says you are "4 weeks pregnant," the embryo is actually about 2 weeks old (counting from fertilization). This 2-week offset continues throughout the entire pregnancy. For a deeper look at how doctors calculate your due date from this starting point, see our guide on how to calculate your due date.
Everything this week is part of your normal menstrual cycle. Pregnancy-specific changes begin after ovulation and conception.
Even though pregnancy has not started yet, week 1 is an excellent time to prepare. If you are actively trying to conceive, the steps you take now can make a meaningful difference. Prenatal vitamins are the single most important action — folic acid reduces the risk of neural tube defects by up to 70 percent, and it needs to be on board before conception occurs.
Ovulation is approaching — in a typical 28-day cycle, it will happen around day 14, during what will be counted as week 2 of pregnancy. Understanding your fertile window and timing intercourse accordingly maximizes your chances of conception.
These are the most impactful things you can do during week 1 if you are hoping to conceive.
If you are trying to conceive, the most important thing you can do this week is take a prenatal vitamin with at least 400 mcg of folic acid (ideally 800 mcg or methylfolate). Neural tube development happens in the very earliest weeks of pregnancy, often before you know you are pregnant. Starting now ensures you are covered.
Knowing your cycle length helps predict ovulation. In a 28-day cycle, ovulation typically occurs around day 14. If your cycles are 30 days, ovulation is more likely around day 16. Cycle tracking apps, basal body temperature, and ovulation predictor kits can all help pinpoint your fertile window.
Talk to your provider about any prescription or over-the-counter medications you take regularly. Some common medications — including certain acne treatments, anti-inflammatory drugs, and herbal supplements — are not recommended during pregnancy. Getting this sorted now avoids scrambling later.
Eat a balanced diet, stay hydrated, get moderate exercise, and aim for 7 to 9 hours of sleep. Limit alcohol and keep caffeine under 200 mg per day. These habits support fertility and create an optimal environment for a potential pregnancy.
Addressing cycle irregularities and medication concerns now sets you up for a smoother pregnancy.
You are at the very beginning — and that means you have the chance to start this journey in the best possible way. Taking a prenatal vitamin, understanding your cycle, and reviewing your health habits now are some of the most impactful things you can do for a future pregnancy.
Next up is week 2, when ovulation occurs and conception becomes possible. If you want to see the full picture of what lies ahead, our 1 week pregnant guide and week-by-week series starting at 3 weeks pregnant cover every stage. For a symptom-by-symptom timeline, our how soon do pregnancy symptoms start guide breaks down exactly what to expect and when.
This guide is for informational purposes only and is not a substitute for professional medical advice. Always consult your healthcare provider with any questions about your pregnancy.
