Gestational Age Calculator — from LMP

Tracking gestational age

Convert weeks + days into specific dates or find out your current gestational age using your LMP. Ideal for coordinating scans and prenatal visits.

Available modes

• Current gestational age Enter LMP and a reference date (today by default) to get weeks + days, trimester and days until due date.
• Target date for a specific week Specify the target weeks + days and receive the estimated date when you will reach that milestone.

Form fields

• LMP: first day of your last menstrual period.
• Reference date lets you check gestational age on another key date (appointment, travel, etc.).
• Weeks and days in "target date" mode, enter the desired combination (e.g. 32 weeks + 4 days).

Interpretation

Estimates depend on accurate LMP and early ultrasound confirmations. Always validate results with your healthcare team.

What is gestational age?

Gestational age is the time elapsed from the first day of the last menstrual period (LMP) to a given date, expressed in complete weeks and days. It is the universal standard in obstetrics for dating pregnancy, scheduling prenatal tests, and estimating the expected due date (EDD). Although conception typically occurs about 2 weeks after LMP, this method persists because it is reproducible and clinically validated. Naegele's rule, proposed in 1812, calculates the EDD by adding 280 days (40 weeks) to the LMP, assuming a regular 28-day cycle.

LMP versus ultrasound: which is more reliable?

LMP is the usual starting point, but its accuracy depends on the woman remembering the date and having regular cycles. A first-trimester ultrasound (weeks 8-13) measures the crown-rump length (CRL) of the embryo with a margin of error of only 3-5 days, making it the most accurate method for dating early pregnancy. The American College of Obstetricians and Gynecologists (ACOG) recommends adjusting the EDD if the first-trimester ultrasound differs from LMP by more than 7 days. In the second trimester, the margin is 7-10 days, and in the third, 2-3 weeks, so it should not be used to redate the pregnancy.

Trimester breakdown

• First trimester (weeks 1-13): All major organs form. This is the most sensitive period to teratogens. Ultrasound in this period is the most accurate for dating pregnancy.
• Second trimester (weeks 14-27): The fetus grows rapidly and the senses develop. Morphology scans and chromosomal anomaly screening are performed.
• Third trimester (weeks 28-40): The fetus gains weight and the lungs mature. Fetal well-being monitoring intensifies. Term delivery is considered between weeks 37 and 42.

Viability and full-term delivery

The fetus is generally considered viable around 24 weeks of gestation, meaning it could survive outside the uterus with intensive neonatal care, though with significant risks. The threshold of viability varies by hospital and available resources. From 34 weeks onward, outcomes improve substantially. A full-term pregnancy falls between weeks 39 and 40, according to the ACOG classification. Births between weeks 37 and 38 are classified as "early term" and may be associated with higher neonatal admission rates.

When is the due date changed?

A first-trimester ultrasound can change the estimated due date if it differs from LMP by more than 7 days. In the second trimester the acceptable margin widens to 10-14 days, and in the third to 21 days. Accurately redating the pregnancy is critical because it determines the right timing for tests such as first-trimester screening (weeks 11-14), the anatomy scan (weeks 18-22), and third-trimester fetal well-being monitoring. An incorrect date can lead to premature interventions or delays in necessary actions.

Prenatal tests by gestational age

The prenatal testing schedule is directly tied to gestational age. Between weeks 11 and 14, the combined first-trimester screening is performed (nuchal translucency measurement, blood work, and serum markers). The anatomy scan is scheduled between weeks 18 and 22 to assess fetal structure. The glucose tolerance test for gestational diabetes is typically ordered between weeks 24 and 28. From week 36 onward, visits intensify with non-stress fetal monitoring and assessment of the baby's presentation.

When to consult your doctor?

Consult your doctor if you do not remember your LMP, have irregular cycles, there is a discrepancy between ultrasound and LMP, you notice vaginal bleeding, contractions before week 37, decreased fetal movements, or fetal growth does not match the estimated weeks.