Third Trimester After IVF: When It Starts and Symptoms

You are somewhere between 28 and 40 weeks pregnant after assisted conception or a long medicated road, the heartburn is unrelenting, and you woke at 3 a.m. convinced something was wrong. If you are searching when does pregnancy third trimester start, the short answer is week 28. This post is a clear, doctor-led separation of what is normal in late pregnancy and what is not, so the next 3 a.m. has fewer questions in it.

The third trimester runs from 28 weeks through delivery. It brings a predictable cluster of symptoms (heartburn, swelling, Braxton-Hicks contractions, sleep collapse) and a short list of red-flag changes that need a same-day call. After IVF, IUI, or years of trying, the instinct to check every twinge is louder than usual, and the line between vigilance and overwhelm is real. The point of this post is to give you the list, the reasoning, and the threshold for picking up the phone.

When does pregnancy third trimester start, and what defines it

ACOG defines the third trimester as week 28 through delivery, counted from the first day of the last menstrual period.¹ NICE and the WHO use the same 28-week starting point. After IVF, gestational age is calculated from the embryo transfer date plus the embryo's age at transfer, and the trimester boundary is drawn against that recalculated date, not the LMP your app suggested before transfer.

The 28-week marker matters because several clinical things shift around it. Viability is well established. The group B streptococcus (GBS) testing window opens at 36 to 37 weeks. Growth scans become more frequent, particularly after assisted conception or in pregnancies with hypertension, prior small-for-gestational-age babies, or twins.

The "term" definitions begin and shape induction conversations. Early term is 37 to 38 weeks and 6 days. Full term is 39 to 40 weeks and 6 days. Late term is 41 to 41 weeks and 6 days. Post-term is 42 weeks and beyond.¹

For couples who came here from infertility, the third trimester is when bodily symptoms eclipse anxiety as the loudest signal. For some readers that is a relief: the body is noisy enough to drown out the loop in the head. For others it triggers a different kind of worry, because every symptom becomes a question for the OB. The PAI anxiety in later pregnancy post is the sibling for the second pattern.

Common symptoms and what is causing them

Most third-trimester symptoms map onto three physiological shifts. Hormones (relaxin and progesterone) loosen connective tissue. Expanded plasma volume (up roughly 40 percent at term) overloads the lymphatics. And the uterus has run out of room.

Heartburn and reflux: Relaxin and progesterone slow the lower oesophageal sphincter, and the uterus pushes the stomach upward. Up to 80 percent of people report meaningful heartburn in the third trimester. It almost always resolves within hours of delivery.

Swelling (oedema) of feet, hands, and face: Plasma volume is up, lymphatic drainage is slowed by uterine pressure on the pelvic vessels, and gravity does the rest. Dependent swelling of the feet is normal. Sudden swelling of face and hands, especially with headache, is not (see red flags below).

Braxton-Hicks contractions: Irregular, painless or mildly uncomfortable tightening of the uterus. They are usually responsive to hydration, a change in position, and emptying the bladder. They are not labour, and they do not damage the pregnancy. They become more frequent in the last few weeks.

Sleep disruption: Almost universal. The supine sleep position becomes uncomfortable after 28 weeks because the gravid uterus partially compresses the inferior vena cava, and the body switches you to a side. Left lateral is the recommended position because it maximises uterine blood flow. Frequent urination, restless legs, and pelvic discomfort all contribute. ACOG recommends left-side sleeping in late pregnancy.

Round ligament and pelvic girdle pain: Relaxin loosens the pelvic joints, which is necessary for delivery and which causes pain in the meantime, particularly in the sacroiliac joints and the pubic symphysis. A Cochrane systematic review found that physiotherapy interventions reduce severity in moderate-to-severe cases.⁶ If walking, rolling in bed, or climbing stairs has become a planning exercise, ask for a referral to a women's health physiotherapist.

Shortness of breath: The diaphragm is pushed upward by 3 to 4 cm. Tidal volume rises, but functional residual capacity falls, and you feel breathless on stairs that did not previously bother you. New, severe, or one-sided shortness of breath is different and needs assessment.

Haemorrhoids and constipation: Slowed gut motility plus iron supplementation plus a uterus pressing on rectal vessels. Fibre, fluids, and a stool softener if your clinician agrees usually help. The haemorrhoids almost always settle postpartum.

Carpal tunnel symptoms: Fluid retention compresses the median nerve at the wrist. Night-time tingling of the thumb, index, and middle fingers is the classic presentation. It resolves postpartum in most people.

Nausea returning in the third trimester: Slowed gastric emptying as the uterus crowds the stomach can revive nausea after the second-trimester reprieve. It is usually benign. Severe vomiting, dehydration, or right-upper-quadrant pain alongside it is not (see HELLP below).

What is normal at each milestone, 28 to 40 weeks

28 to 32 weeks: Formal kick counts begin per ACOG.² The GTT, if not already done, is scheduled at 24 to 28 weeks. Anti-D immunoglobulin is given to people who are Rh-negative. Visits move from monthly to fortnightly in most units.

32 to 36 weeks: A growth scan is performed if indicated by your history (IVF or IUI, twins, prior small-for-gestational-age baby, hypertension, gestational diabetes). The baby usually settles into a vertex (head-down) position by 36 weeks; if not, an external cephalic version may be offered.

36 to 37 weeks: GBS swab in US practice. NICE uses a risk-factor-based approach in the UK rather than universal swabbing. A cervical check is sometimes offered; the evidence for routine cervical checks at this stage is weak, and you can decline without compromising your care.

38 to 40 weeks: "Nesting" energy returns for many people. Lightning crotch (a sudden sharp pelvic nerve pain) often appears as the fetal head engages. Braxton-Hicks contractions intensify. You may lose the mucus plug. Visits become weekly.

40 weeks and beyond: The induction conversation typically begins at 41 weeks per ACOG and is offered earlier in high-risk pregnancies, including some IVF pregnancies depending on the indication. Amniotic fluid and biophysical profile monitoring increase if you go past your due date.

Kick counts, the right way

Formal kick counting starts at 28 weeks. Earlier counting tends to produce false alarms because fetal movement is not yet regular enough to follow a daily pattern.²

The protocol used in many NHS units and in the "Count the Kicks" framework is: at the same time each day, lie on your left side, and count distinct fetal movements. Most pregnancies reach 10 movements within 30 minutes. ACOG accepts up to 2 hours. Note how long it took, not just whether you reached 10. The trend in your own pattern is what matters; the absolute number varies.

Decreased fetal movement is one of the strongest single predictors of stillbirth in late pregnancy. If movement is reduced compared with your usual pattern, do not wait it out, do not drink cold water and hope for the best, and do not text a friend for reassurance. Call triage. After IVF, IUI, or a long TTC story, do not feel embarrassed about the call; reduced movement is the most evidence-based reason to come in for monitoring, and triage units are set up for this. Many people will call several times across a third trimester, and all those calls are appropriate.

Red flags that need a same-day call or a triage visit

This is the working list I give patients in late pregnancy. Any one of these is a call.

• No fetal movement after stimulation (eat, drink cold water, lie on left side, give it an hour). Call same day.
• Vaginal bleeding more than spotting.
• Persistent severe headache, especially with visual changes (flashing lights, blurring, a curtain effect). Preeclampsia is the diagnosis being ruled out.⁷
• Right-upper-quadrant or epigastric pain: HELLP syndrome (haemolysis, elevated liver enzymes, low platelets) presents this way and can develop fast.
• Sudden swelling of face or hands, especially with headache. Preeclampsia again.
• Fluid leakage from the vagina: Premature rupture of membranes (PROM) is sometimes obvious (a pop and a gush) and sometimes a slow leak that is mistaken for urine. The unit can confirm with a swab.
• Regular contractions every 5 minutes for an hour before 37 weeks: Preterm labour assessment.
• Severe itching of palms or soles, especially at night: Intrahepatic cholestasis of pregnancy needs bile acid testing per the RCOG Green-top 43 guideline, because severe disease is associated with stillbirth.³
• Fever above 38°C (100.4°F).
• Calf swelling, redness, or asymmetry: DVT risk is meaningfully elevated in late pregnancy and the postpartum window.

The list looks long, and that is the point. Most of these will not be happening to you. Knowing them in advance is what lets you ignore the noise that is not on the list, and act fast on the noise that is.

After IVF or IUI specifically

A few patterns are worth knowing about for the conception-after-assisted-reproduction reader.

Singleton ART pregnancies carry a modestly elevated rate of hypertensive disorders of pregnancy, gestational diabetes, and placental anomalies compared with spontaneously conceived pregnancies.⁴ In numerical terms, the absolute increases are small, but they are real, and they are part of why your scan and BP visit schedule is likely more frequent. Do not skip BP visits because you feel fine; preeclampsia is often asymptomatic in its early phase.

Frozen embryo transfer (FET) pregnancies carry a slightly higher hypertensive-disorder risk than fresh transfers in the cumulative meta-analytic data.⁵ The biology is debated (the absent corpus luteum in programmed cycles is one hypothesis), but the implication for monitoring is the same: BPs are not optional in the third trimester.

IVF singletons have a small absolute increase in preterm birth rates. Paying close attention to contraction patterns in the late second trimester and the early third trimester is worth doing. The threshold for calling triage about contractions, after IVF, is appropriately lower than for a spontaneous pregnancy.

What you can do tonight

Concrete and proportionate.

• For heartburn: Smaller meals, head of bed elevated by 4 to 6 inches, avoid lying flat for 2 hours after eating. Calcium carbonate antacids are pregnancy-safe and can be discussed with your provider for night-time symptoms.
• For sleep: Left lateral position with a pillow between the knees, a pregnancy wedge or full-body pillow if that helps, dim screens an hour before bed. Melatonin in pregnancy is not formally recommended, and the safety data are limited; discuss with your provider before using it.
• For Braxton-Hicks: Drink a large glass of water, change position, empty the bladder. If contractions continue rhythmically every 5 minutes for over an hour before 37 weeks, that crosses into the call category.
• For swelling: Elevate feet three times a day, compression stockings, hydrate (counterintuitive but it works), reduce salt only if your BP is rising. Sudden facial or hand swelling is different and is on the red-flag list.
• For anxiety about movement: A daily kick-count practice at the same time each day. Data calms the brain; constant ad-hoc checking feeds the anxiety.

What to ask at every late-pregnancy visit

• "What is my blood pressure trend across the last three visits?"
• "When should we discuss induction timing given my history?"
• "Is my growth tracking on the same curve, or has it shifted?"
• "What position is the baby in today, and when do you expect that to settle?"
• "What is your unit's policy on after-hours triage calls? Should I call first, or come in?"

The visits are short and the lists are long. From when does pregnancy third trimester start through to delivery, writing the questions down in advance is the difference between leaving with answers and leaving with a vague reassurance.

What's next

• For the physical context behind these symptoms: second trimester changes
• If anxiety is the dominant feature: PAI anxiety in later pregnancy
• If you are writing the birth plan: birth plan after IVF or IUI
• If a cesarean is on the table: cesarean after IVF
• If something feels off: when things don't go to plan, late pregnancy concerns

Sources

1. American College of Obstetricians and Gynecologists. Committee Opinion No. 579: Definition of term pregnancy. Obstet Gynecol 2013;122(5):1139-1140 (reaffirmed 2022). https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2013/11/definition-of-term-pregnancy
2. American College of Obstetricians and Gynecologists. Practice Bulletin No. 234: Antepartum fetal surveillance. Obstet Gynecol 2021;138(2):e26-e35. https://www.acog.org/clinical/clinical-guidance/practice-bulletin/articles/2021/06/antepartum-fetal-surveillance
3. Royal College of Obstetricians and Gynaecologists. Intrahepatic cholestasis of pregnancy (Green-top Guideline No. 43). RCOG; 2022. https://www.rcog.org.uk/guidance/browse-all-guidance/green-top-guidelines/intrahepatic-cholestasis-of-pregnancy-green-top-guideline-no-43/
4. Vermey BG, Buchanan A, Chambers GM, et al. Are singleton pregnancies after assisted reproduction technology (ART) associated with a higher risk of placental anomalies compared with non-ART singleton pregnancies? A systematic review and meta-analysis. BJOG 2019;126(2):209-218. https://obgyn.onlinelibrary.wiley.com/doi/10.1111/1471-0528.15227
5. Maheshwari A, Pandey S, Amalraj Raja E, Shetty A, Hamilton M, Bhattacharya S. Is frozen embryo transfer better for mothers and babies? Can cumulative meta-analysis provide a definitive answer? Hum Reprod Update 2018;24(1):35-58. https://academic.oup.com/humupd/article/24/1/35/4569360
6. Liddle SD, Pennick V. Interventions for preventing and treating low-back and pelvic pain during pregnancy. Cochrane Database Syst Rev 2015;(9):CD001139. https://www.cochranelibrary.com/cdsr/doi/10.1002/14651858.CD001139.pub4/full
7. American College of Obstetricians and Gynecologists. Gestational hypertension and preeclampsia: ACOG Practice Bulletin No. 222. Obstet Gynecol 2020;135(6):e237-e260. https://www.acog.org/clinical/clinical-guidance/practice-bulletin/articles/2020/06/gestational-hypertension-and-preeclampsia