When does second trimester start

when does second trimester start

Pregnancy is grouped into three stretches called trimesters. The American College of Obstetricians and Gynecologists defines the first as the first day of the last menstrual period through 13 weeks 6 days, the second as 14 weeks 0 days through 27 weeks 6 days, and the third as 28 weeks 0 days through 40 weeks 6 days. A typical dating clock for a singleton pregnancy is about 40 weeks from that last period, which already includes about two weeks before fertilization.

Some popular charts treat week 13 as the opening of the middle stretch. Clinics that follow the college’s cut use 14 weeks 0 days. If two apps disagree by a week, they are using two fences, not two pregnancies. Ask which fence your clinician is using and write that down. The start you need for tests and visit timing is the one on the medical chart.

Think of the trimesters as seasons of a pregnancy: an opening season, a middle season, a closing season. They are not January, April, and July. A person who conceives in November may enter the second trimester in winter. A person who conceives in May may enter it in late summer. The calendar season is scenery. The pregnancy season is the week number.

Use the weeks on the chart, counted from the last menstrual period unless the clinician redated from ultrasound or from assisted reproduction. Treat 14 weeks 0 days as the ordinary obstetric start of the second trimester.

Treat 27 weeks 6 days as the last day of that trimester and 28 weeks 0 days as the start of the third. Do not convert those weeks into a month of the year and call that month the start.

Weeks, not weather

Heat, cold, and daylight can change how the middle stretch feels. They do not start it. Nausea that eases in week 14 in January and nausea that eases in week 14 in July are the same obstetric season in two weathers. Energy that returns for many people in the middle stretch is a common pattern, not a law, and not a function of spring. If you feel worse, the season outside is not the diagnosis.

Because a year has four meteorological seasons and a pregnancy has three trimesters, they will not stay aligned. Do not wait for the equinox to “enter the second trimester.” Do not treat a heat wave as proof that you have arrived. The fence is a week count. Crossing it does not require leaves on the trees.

The middle of a pregnancy is a week number. Spring is optional scenery. If dates were adjusted after an early ultrasound, the second-trimester start moves with that adjustment. The rule is still 14 weeks 0 days on the dating you are actually using. Twin pregnancies and other clinical situations can change visit schedules without changing what the word trimester means. For care questions, the clinician in front of you outranks a webpage.