Pregnancy Trimester Calculator

Enter Calculation

Gestational Age (weeks)

Additional Days (0-6)

Formula

Show calculation steps (2)

1. Weeks Remaining to Full Term

   

   Weeks left until the 40-week full term (floored at 0)

2. Trimester

   

   Trimester is set by completed gestational week

Results

Current Trimester

1

First Trimester (weeks 1-13)

Gestational Age	12 weeks
Weeks Remaining (to 40)	28 weeks

What Is the Pregnancy Trimester Calculator?

Pregnancy is conventionally divided into three trimesters, each marking a distinct phase of fetal development and maternal change. This calculator converts your gestational age — measured in completed weeks (and optional days) since the first day of your last menstrual period — into the corresponding trimester. It also shows how many weeks remain until the 40-week full-term milestone.

How to Use It

Enter your current gestational age in weeks (0–42) and, if you know it, the additional number of days (0–6). The calculator instantly returns your trimester, your total gestational age, and the weeks remaining to term. Gestational age is the standard clinical measure used by midwives and obstetricians worldwide.

The Formula Explained

The trimester boundaries follow the common clinical convention: the first trimester spans weeks 1 through 13, the second trimester spans weeks 14 through 27, and the third trimester spans weeks 28 through 40. The calculator compares your completed weeks against these thresholds. Weeks remaining is simply 40 minus your total gestational age, where total weeks = weeks + days ÷ 7.

$$\text{Total} = \text{Weeks} + \frac{\text{Days}}{7}$$

$$\text{Remaining} = 40 - \left(\text{Weeks} + \frac{\text{Days}}{7}\right)$$

$$\text{Trimester} = \begin{cases} 1 & \text{Weeks} \le 13 \\ 2 & 14 \le \text{Weeks} \le 27 \\ 3 & \text{Weeks} \ge 28 \end{cases}$$

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The 40 weeks of pregnancy divided into three trimesters: weeks 1-13, 14-27 and 28-40.

Worked Example

Suppose you are 20 weeks and 3 days pregnant. Since 20 falls between 14 and 27, you are in the second trimester. Your total gestational age is \(20 + 3/7 \approx 20.43\) weeks, leaving about \(40 - 20.43 = 19.57\) weeks until term.

FAQ

Are trimester boundaries exact? No — different sources use slightly different cutoffs (some end the first trimester at week 12). This tool uses the widely taught 13/27/40 boundaries.

What if I'm past 40 weeks? You remain in the third trimester; weeks remaining is shown as 0 since you have reached full term.

Is this a substitute for medical advice? No. Always confirm dates and any health concerns with your healthcare provider.