Pregnancy Week Calculator

How to Calculate How Far Along You Are

Gestational age — how far along you are in pregnancy — is measured in weeks and days from the first day of your last menstrual period (LMP). This means pregnancy is officially "40 weeks long," even though conception typically doesn't occur until around week 2.

The reason clinicians count from LMP rather than conception is practical: the start of a period is easy to remember, while the exact moment of conception is rarely known with certainty. This LMP-based dating system is the universal standard used by OB-GYNs worldwide and is recommended by ACOG.

What Each Trimester Looks Like

Pregnancy is divided into three roughly equal phases, each marked by distinctive developmental milestones and clinical events.

First Trimester (Weeks 1–13)

The first trimester spans from your LMP through the end of week 13. During these 13 weeks, the fertilized egg implants in the uterine wall, major organ systems begin forming, and the embryo transitions to a fetus by week 10. The heartbeat is typically visible on ultrasound by weeks 6–7.

Many people experience their most intense pregnancy symptoms during this period — nausea, fatigue, breast tenderness, and food aversions. The miscarriage rate is also highest in the first trimester, falling significantly after the heartbeat is confirmed.

Key first-trimester appointments include the first prenatal visit (typically weeks 8–10) and first-trimester screening for chromosomal conditions (weeks 11–13).

Second Trimester (Weeks 14–27)

The second trimester begins at week 14 and runs through week 27. For many, this is the most comfortable stretch of pregnancy. Morning sickness usually fades, energy levels improve, and the visible pregnancy bump appears. Most people feel their baby move for the first time (called quickening) between weeks 16 and 22.

The anatomy scan, typically performed around week 20, is a detailed ultrasound that checks fetal organs, spine, limbs, and (if desired) fetal sex. Glucose screening for gestational diabetes is done around weeks 24–28.

Third Trimester (Weeks 28–40)

The third trimester spans weeks 28 through 40 (and sometimes beyond). During these weeks the baby gains the majority of its birth weight — roughly half a pound per week in the final stretch. The baby's position also becomes more important as the due date approaches.

Prenatal visits become more frequent in the third trimester, often weekly from week 36 onward. A Group B Streptococcus (GBS) test is performed around week 36. Most providers won't recommend induction before 39 weeks without a medical reason.

Baby's Development Week by Week

Each week of pregnancy brings measurable changes in fetal size and capability. Here are a few landmark weeks:

• Week 6: Heart begins beating. Embryo is approximately the size of a lentil (~0.25 inches).
• Week 10: All major organ systems have formed; embryo becomes a fetus. Fingers and toes are distinct.
• Week 14: Kidneys are producing urine. Baby can make facial expressions. Size: a lemon.
• Week 20: Halfway point. Baby can hear sounds from outside the womb. Size: a banana (~10 inches head to heel).
• Week 24: Viability threshold. With intensive NICU care, survival is possible. Lungs are developing surfactant.
• Week 28: Eyes open for the first time. Brain is developing rapidly. Size: an eggplant.
• Week 34: Fat stores building. Fingernails and toenails fully formed. Size: a butternut squash.
• Week 37–40: Full term. Baby is ready for delivery.

Frequently Asked Questions

• Why does pregnancy "start" before I was actually pregnant?

  Because gestational age is counted from the first day of your last period — about two weeks before conception. This is a clinical convention that standardizes pregnancy dating worldwide. Fetal age (counted from conception) is approximately 2 weeks less than gestational age. When your provider says "you're 10 weeks pregnant," they mean 10 weeks gestational age, or roughly 8 weeks post-conception.

• How accurate is week-based pregnancy dating?

  LMP-based dating is a solid estimate but assumes a 28-day cycle with ovulation at day 14. Ultrasound dating — especially a first-trimester scan — is more precise for individual pregnancies. Your provider may revise your gestational age if the ultrasound measurements don't align with the LMP date.

• I'm past 40 weeks — am I overdue?

  "Full term" is 39 weeks 0 days through 40 weeks 6 days. "Late term" is 41 weeks 0 days through 41 weeks 6 days. "Post term" is 42 weeks and beyond. Going past 40 weeks is common — only about 5% of babies arrive on their exact due date. Discuss induction options with your provider starting around 41 weeks.

• What's the difference between gestational age and fetal age?

  Gestational age counts from LMP (the start of your last period), while fetal age counts from the actual date of fertilization. Gestational age is always approximately 2 weeks more than fetal age. Healthcare providers universally use gestational age because it's more reliable and consistent.