Pregnancy Weeks Converter

A pregnancy weeks converter translates gestational age from weeks into months, trimesters, days, and other useful time units. Pregnancy dating follows a standardized system used by healthcare providers worldwide: gestational age is measured from the first day of the last menstrual period (LMP), which is approximately two weeks before conception actually occurs. This convention, while somewhat counterintuitive, provides a consistent reference point that all obstetricians and midwives can agree upon.

A typical human pregnancy lasts approximately 40 weeks (280 days) from the LMP, divided into three trimesters. The first trimester spans weeks 1–12 and involves critical organ development, the formation of the placenta, and the transition from embryo to fetus. The second trimester (weeks 13–26) is characterized by rapid growth, the development of senses, and often the most comfortable period for the mother. The third trimester (weeks 27–40) involves final lung maturation, significant weight gain, and preparation for birth.

Understanding gestational weeks is important because medical appointments, prenatal screenings, and developmental milestones are all referenced by gestational week. An anatomy scan is typically performed at 18–22 weeks, glucose screening at 24–28 weeks, and Group B Strep testing at 36–37 weeks. Knowing your gestational age helps you understand where you are in your pregnancy journey and what to expect at each stage.

This converter allows you to enter your gestational age in weeks and days, optionally providing your LMP date for due date calculation. It displays the corresponding months, trimester, days remaining, baby size comparisons, and upcoming developmental milestones.

Pregnancy calculations are based on the standard obstetric convention that gestational age starts from the first day of the last menstrual period (LMP). The estimated due date (EDD) is calculated using Naegele's rule: add 280 days (40 weeks) to the LMP date. This gives an estimated due date that has a 5% chance of being the actual birth date — most babies arrive within 2 weeks before or after this date.

The conversion from weeks to months uses two different methods. Lunar months divide total gestational weeks by 4 (since each lunar month is exactly 4 weeks), giving a simple and consistent result. Calendar months divide total gestational days by 30.44 (the average number of days in a month), providing a result that aligns more closely with how people think about months in everyday life.

The calculator also tracks trimester boundaries: the first trimester ends at week 12, the second at week 26, and the third at week 40. Within each trimester, the calculator shows the week number relative to that trimester's start (e.g., week 15 is "3 weeks into the second trimester"). This is useful for understanding your position within the broader pregnancy timeline.

For the due date calculation from LMP, the formula is: Due Date = LMP + 280 days. The calculator also computes the current gestational age from LMP by calculating the number of days between today and the LMP date, then converting to weeks and remaining days.

This calculator provides comprehensive pregnancy tracking from your gestational age:

1. Enter Weeks Pregnant: Type the number of completed weeks (0–42) into the input field.
2. Add Days: Use the dropdown to add 0–6 additional days beyond the completed weeks.
3. Enter LMP Date (Optional): If you know your last menstrual period date, enter it to calculate your estimated due date and current gestational age from LMP.
4. View Gestational Age: The main display shows weeks and days, the trimester number, and baby's approximate size.
5. Review Conversion Details: The results show total days, calendar months, lunar months, and days remaining until the due date.
6. Check Due Date: If LMP was entered, the estimated due date is displayed along with the current gestational age from LMP.
7. Track Milestones: The upcoming milestones section shows the next developmental events in your pregnancy.
8. Use Quick Select: Click any week number (1–41) to jump directly to that week's information.

The calculator displays several categories of results. The Gestational Age section shows weeks and days, the trimester number, and baby's approximate size compared to common fruits and vegetables. For example, at 20 weeks, the baby is approximately the size of a banana.

The conversion results include total days pregnant, calendar months (using 30.44 days per month), lunar months (using 4 weeks per month), and days remaining until the 280-day (40-week) due date. The calendar month conversion provides a more intuitive sense of "how far along" you are in familiar monthly terms.

The upcoming milestones section shows the next 3 developmental events in your pregnancy, such as the anatomy scan (week 20), viability milestone (week 24), or full-term begins (week 37). These milestones help you understand what to expect at upcoming prenatal visits and stages of development.

The weeks-to-months conversion table provides a quick reference showing which weeks correspond to which months and trimesters. Weeks 1–4 correspond to month 1 (1st trimester), weeks 5–8 to month 2, and so on through weeks 36–40 corresponding to month 9 (3rd trimester).

Pregnancy week conversion is essential for prenatal healthcare. Medical appointments, screenings, and tests are all scheduled by gestational week. The first ultrasound is typically at 8–12 weeks, the nuchal translucency screening at 11–14 weeks, the anatomy scan at 18–22 weeks, and glucose tolerance testing at 24–28 weeks. Understanding your gestational age ensures you attend these critical appointments on time and understand the purpose of each test.

Baby registries and shopping are organized by pregnancy month. Many expecting parents start purchasing items during the second trimester (months 4–6), when they know the baby's sex and have more energy. Understanding the month-to-week correspondence helps parents plan their shopping timeline and prepare the nursery at the appropriate stage.

Fetal development tracking depends on accurate gestational age. Baby size comparisons (e.g., "your baby is the size of a lemon at 14 weeks") help parents visualize their baby's growth. Developmental milestones such as when the baby's eyes can open (week 28), when the lungs are developing rapidly (week 32), and when full-term begins (week 37) provide meaningful markers of progress.

Workplace planning benefits from accurate pregnancy week calculations. Understanding when you enter the third trimester helps with planning maternity leave, transitioning responsibilities, and communicating with employers about expected timeline. Many women reduce their workload during the third trimester as physical demands increase.