Pregnancy Trimester by Trimester: A Partner's Guide to 40 Weeks

Pregnancy can feel like something that is happening near you rather than to you. Your partner feels every change first, and you mostly hear about it secondhand. But the partner who understands what each stage involves can plan ahead, show up at the right moments and take real weight off the person who is carrying the baby.

This guide walks through pregnancy the way obstetricians count it, trimester by trimester, with what is going on for the baby, what your partner may be feeling, and where you fit in. It is general education, not medical advice. Your partner's obstetric provider is the right source for anything specific to your pregnancy.

How the weeks are counted

The American College of Obstetricians and Gynecologists (ACOG) explains that a normal pregnancy lasts about 40 weeks from the first day of the last menstrual period, or LMP. Pregnancy is assumed to start two weeks after that date, so the count includes about two weeks when no one was actually pregnant yet. That is why the week number your partner hears at a visit can seem "ahead" of when you think conception happened.

ACOG defines the trimesters this way:

• First trimester: from the first day of the LMP to 13 weeks and 6 days
• Second trimester: 14 weeks and 0 days to 27 weeks and 6 days
• Third trimester: 28 weeks and 0 days to 40 weeks and 6 days

Treat the due date as a target, not an appointment. ACOG notes that only about 1 in 20 women give birth on their due dates. March of Dimes explains that ACOG and the Society for Maternal-Fetal Medicine call a birth between 39 weeks, 0 days and 40 weeks, 6 days "full term," and a birth between 37 weeks, 0 days and 38 weeks, 6 days "early term."

First trimester: big changes nobody can see

According to ACOG, the brain and spine begin to form and cardiac tissue starts to develop in weeks 1 to 8. By weeks 9 to 12, the kidneys begin making urine and fingernails form. From the outside, almost none of this shows.

What your partner may notice instead is nausea and exhaustion. ACOG says nausea and vomiting of pregnancy usually starts before 9 weeks and, for most women, goes away by 14 weeks, though for some it lasts longer. March of Dimes lists fatigue as common early and late in pregnancy, partly because the body is working hard and making pregnancy hormones.

Where you fit in:

• Encourage an early first prenatal visit. March of Dimes advises calling a provider to schedule the first checkup as soon as the pregnancy is known.
• Gather your own family health history. March of Dimes notes that providers ask about the health conditions of the partner and both families, not just the pregnant person.
• Take on chores that trigger nausea. ACOG suggests having someone else empty the trash when odors are a problem.
• Ask to come along. March of Dimes says a partner or support person is welcome at prenatal checkups, and the first visit usually includes a conversation about which prenatal tests to consider.

Second trimester: growth you can start to share

ACOG describes the second trimester as a time of rapid growth and development. Hearing starts to develop in weeks 13 to 16. By weeks 17 to 20, the ears, nose and lips are recognizable on an ultrasound exam, and by weeks 21 to 24, kicks and turns are stronger.

This can be when the pregnancy starts to feel real for you. March of Dimes says most women get an ultrasound at 18 to 20 weeks, and that someone in a first pregnancy may feel the baby move by about 20 weeks.

There is also a routine test to plan around. March of Dimes says a glucose screening test for gestational diabetes is done between 24 and 28 weeks.

Where you fit in:

• Go to the anatomy ultrasound if you can, and ask your partner beforehand whether you both want to learn the baby's sex. March of Dimes advises telling the provider if you don't want to know.
• Use the extra energy many people have in this stretch to plan: childbirth classes, the car seat, the nursery and leave from work.
• Keep checking in on how your partner is feeling, physically and emotionally, rather than assuming the hard part is over.

Third trimester: the home stretch

In the third trimester, ACOG says the fetus gains weight and the organs mature so they will be ready to function after birth. In weeks 33 to 36, the fetus may turn into a head-down position, and by weeks 37 to 40 the lungs, brain and nervous system are finishing their development.

Visits pick up speed. March of Dimes describes a typical schedule of one checkup every four weeks through week 28, every two weeks from 28 to 36, and weekly from 36 to 41, with more frequent visits if there are complications. Late in pregnancy, the provider also feels the belly to check the baby's position and may test for group B strep.

Physical discomfort often peaks here. March of Dimes lists back pain, heartburn, leg cramps, swelling, shortness of breath and needing to urinate often as common later in pregnancy. Our guide Nausea, Fatigue and Aches: How to Help Your Pregnant Partner covers practical ways to help.

Where you fit in:

• Do the labor logistics. ACOG suggests rehearsing the drive to the hospital, packing a bag ahead of time, arranging care for other children and pets, and making sure the car seat is installed properly.
• Ask the care team, well before the due date, when to call, how to reach them after hours, and whether to call first or go straight to the hospital.
• Learn the warning signs that need fast care, during pregnancy and in the year after birth. Our guide Urgent Warning Signs: When Pregnancy or Postpartum Needs Fast Care lists them.

Your own feelings count too

You may be surprised by your own swings of excitement, worry and doubt. Pregnancy affects the whole household, and paying attention to your own wellbeing is part of taking care of your family.

Keep an eye on your partner's mood as well as their body. March of Dimes describes perinatal depression, which can occur during pregnancy or in the first year after birth, as one of the most common medical complications of pregnancy, affecting up to 1 in 7 women. The National Maternal Mental Health Hotline, run by the Health Resources and Services Administration, is free, confidential and available 24/7 by call or text at 1-833-TLC-MAMA (1-833-852-6262), and it encourages partners and family members to reach out too. If anyone is thinking about self-harm, call or text 988. For a medical emergency, call 911.

Key takeaways

• Pregnancy is counted as about 40 weeks from the first day of the last menstrual period, split into three trimesters.
• Only about 1 in 20 births happen on the due date, so plan for a window, not a day.
• The first trimester is often the hardest physically even though little shows; practical help with chores and nausea triggers matters.
• The second trimester brings the anatomy ultrasound, first movements and the glucose screening.
• In the third trimester, visits become more frequent and it is time to finish logistics such as the hospital route, the bag and the car seat.
• Watch moods as well as symptoms, and know the hotline numbers for both of you.

Sources

• American College of Obstetricians and Gynecologists, How Your Fetus Grows During Pregnancy (last reviewed March 2026)
• American College of Obstetricians and Gynecologists, Morning Sickness: Nausea and Vomiting of Pregnancy (last reviewed November 2025)
• American College of Obstetricians and Gynecologists, How to Tell When Labor Begins (last reviewed November 2025)
• March of Dimes, Prenatal care checkups
• March of Dimes, What is full-term?
• March of Dimes, Common discomforts of pregnancy
• March of Dimes, Depression during pregnancy
• Health Resources and Services Administration, Maternal and Child Health Bureau, National Maternal Mental Health Hotline